Start with the trunking user and the selected PBX handoff
OnSIP implements trunking by configuring a user on its Hosted PBX platform to route calls to and from the customer's PBX. Enabling the feature creates authentication information for that PBX. This is not the same operating model as using OnSIP's hosted call features directly.
Record the exact account, trunking user, PBX version, registrar and proxy values, transport, authentication owner, number inventory, capacity and support boundary from the protected implementation record. OnSIP says it now offers SIP trunking only; do not carry forward an older IAX assumption.
Documented service facts
Reviewed 2026-10-09. Compare SIP trunk services →
SIP authentication
DocumentedOnSIP says enabling trunking on a hosted-PBX user creates authentication information for the customer's PBX. The reviewed public pages do not expose a safe, complete current credential schema, digest realm or algorithm, secret rotation process, source-address policy, certificate role or allowlist. Record the account-generated values and their owners in a protected implementation handoff; never copy reusable credentials into tickets or captures.
SIP registration
DocumentedOnSIP documents a user configured to route calls to and from the customer's PBX and notes that the trunking flag is stored with the registration until the device re-registers. That does not establish every PBX's registrar, proxy, transport, Contact, expiry, qualification, DNS or retry settings. Use the current account record and the applicable PBX configuration guide, then validate a sanitized registration exchange.
Inbound delivery
DocumentedOnSIP documents transferring existing PSTN Gateway numbers to the hosted-PBX account and warns that transferred numbers begin ringing the new PBX immediately. It also documents provider-side ring time and external-number failover for the trunking user. These facts do not establish one account's assigned numbers, completed ports, Request-URI, asserted identity, digit format or PBX route. Reconfigure first, preserve the number record and validate each accepted inbound pattern.
Routing and failover
DocumentedOnSIP documents rerouting an inbound call to a selected external number after a configured ring time and warns that PBX inbound rules can conflict with or pre-empt this preference. This is a provider-side new-call option, not proof of access, PBX, registration or media-path diversity. Record the trigger, timer, destination, loop prevention, caller experience, established-call behavior and restoration order, then exercise an approved failure case.
Number and regional availability
Not verifiedThe reviewed pages establish that OnSIP can transfer existing PSTN Gateway numbers and that paid business plans provide PSTN gateway service; they do not establish current number availability, portability, serviceability or lead time for a requested rate centre or address. Retain dated qualification and port responses for every number block and activation date. Do not confuse a free SIP address with an orderable PSTN number.
Audio codecs and DTMF
Not verifiedThe reviewed trunking pages do not publish one complete current codec, packetization, DTMF, early-media, transcoding or media-address profile. Obtain the current OnSIP handoff for the selected PBX and preserve both SDP bodies, negotiated payload mapping, RTP evidence and decoded digits for every required call path. A PBX-specific setup article is not proof of negotiation on the production call.
T.38 fax
Not verifiedThe reviewed trunking records do not establish T.38 or audio-fax entitlement, direction, switchover behavior, gateway compatibility, redundancy or page limits. Treat fax, alarm, elevator and other non-voice devices as separate workflows with their own service confirmation, path, power, monitoring and acceptance evidence.
Signaling and media encryption
Not verifiedThe public Free Plan page discusses encrypted peer-to-peer media when both parties use the OnSIP app, but that is not a trunk security profile and must not be generalized to PSTN or PBX call legs. The reviewed trunking pages do not establish SIP TLS versions, certificate validation, client certificates, SRTP suites, keying or protected legs for one trunk. Verify signaling and media protection separately on the selected route.
Concurrent calls and call starts
Not verifiedThe reviewed first-party support pages do not establish the selected account's simultaneous-call allocation, calls per second, burst behavior, codec bandwidth, registration scale or PBX resource limits. Bind each numeric limit to the current order and an approved load plan. A number's ability to receive multiple calls or a plan description does not prove account, access or PBX capacity.
Emergency calling
DocumentedOnSIP explicitly says a user configured for hosted trunking cannot use its other hosted-PBX features including E911, and its emergency-location article says E911 is not available to Hosted Trunking subscribers. Design and contract a separate compliant emergency-calling path before service activation; map every fixed and nomadic location, callback identity, notification and outage case, coordinate only permitted tests, and retain an independent means of reaching emergency services.
Pricing model
Not verifiedPublic marketing and support records do not constitute the selected account's accepted price. Bind account minimums, numbers, usage, international and toll-free rates, porting, failover destinations, emergency alternatives, taxes, fees, support, term, renewal and termination to a dated quote, contract and bill. Recheck before purchase because plan names and rates can change.
Administration and support
DocumentedOnSIP documents enabling trunking on a hosted-PBX user, selecting phone-provided outbound caller ID, using the 000 prefix and configuring inbound failover in the admin application. Its support catalogue lists PBX-specific trunking articles. These procedures do not establish one account's roles, monitoring, change approval, escalation, response target or service credit. Preserve authorized changes, tickets and timestamps, and re-register deliberately when clearing the trunking flag.
SMS and MMS
Not verifiedThe reviewed trunking records do not establish SMS or MMS entitlement on trunk numbers, sender registration, API access, throughput, webhook security, retention or price. The Free Plan's messaging and SIP-address capabilities are a different product context and must not be treated as trunk-number messaging. Require a separate current service record and workflow.
Treat the 000 prefix as a deliberate routing dependency
OnSIP's enabling guide says all trunking calls require a 000 prefix to select trunking-style delivery. Put that rule in the controlled outbound-routing design rather than asking users to remember it. Preserve emergency and service-code routes before any generic transformation, normalize ordinary destinations once, and prevent recursive prefixing.
Validate sanitized examples for local, national, international, toll-free, emergency and blocked patterns against the current handoff. A successful registration does not prove that the PBX created the required Request-URI or presented an authorized calling identity.
Sequence number transfer around the cutover
OnSIP warns that an existing PSTN Gateway number begins ringing the new PBX as soon as it is transferred to the hosted-PBX account. Reconfigure and validate the new PBX before that administrative move.
Maintain a number-level cutover ledger with the old and new owner, expected digit form, route, transfer time, test result, rollback authority and support case. The public article describes a mechanism; it does not prove that a particular port is complete or that every inbound identity pattern reaches the intended destination.
Use inbound failover without hiding the actual failure
OnSIP documents a provider-side ring timer followed by rerouting to a selected external number. It also warns that customer-PBX inbound rules can conflict with or pre-empt that preference. This option can protect selected new calls, but it does not make the customer access, PBX, registration and media path diverse.
Record the trigger, timer, destination, caller experience, loop prevention, voicemail interaction, emergency implications, established-call behavior and restoration order. Exercise an approved new-call failure case and correlate the provider preference with PBX and destination evidence.
Do not assign OnSIP E911 to a trunking user
OnSIP explicitly states that a user configured for hosted trunking cannot use hosted-PBX features including E911, and its emergency-location documentation says E911 is not available to Hosted Trunking subscribers. This is an exclusion, not an unverified optional feature.
Before activation, design and contract a separate compliant emergency path. Map dispatchable locations, nomadic behavior, callback identity, notifications, power and network outages, failover destinations and permitted test procedures. Maintain an independent means of reaching emergency services and never infer emergency readiness from ordinary inbound or outbound success.
Keep the Free Plan and app claims outside the trunk record
OnSIP's Free Plan supplies SIP addresses for app or device registration, SIP-to-SIP calling and messaging, while PSTN calling requires paid gateway service. Its app-to-app security description also does not establish protection on PBX, carrier or PSTN legs.
Do not use Free Plan user counts, active registrations, messaging or peer-to-peer encryption as evidence for a paid trunk. Close codecs, DTMF, media addressing, fax, TLS, SRTP, number messaging, capacity, serviceability, compatibility, support and price from the selected order, current handoff and authorized traces.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- OnSIP Trunking ↗
- Enabling OnSIP Trunking ↗
- OnSIP Trunking inbound call preferences ↗
- OnSIP E911 emergency response locations ↗
- OnSIP Support ↗
- OnSIP Free Plan ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.