Identify the country, order, trunk set and PBX endpoints
Record the NFON contracting country, legal account, Basic or Premium feature set, ordered channels, number blocks, trunk set, each PBX endpoint, template, support owner and any optional services. A PBX endpoint describes a customer-side SIP target; number routing separately associates called-number prefixes with a trunk set.
Do not infer the current design from an old device template or a working outbound call. Preserve the portal export, order, endpoint activation, number-routing objects and rollback owner before changing authentication or high availability.
Documented service facts
Reviewed 2026-10-12. Compare SIP trunk services →
SIP authentication
DocumentedNFON documents PBX-endpoint username and password authentication and an alternative static mode in which the customer supplies a public static address. These are selectable service modes, not cumulative requirements. Preserve the selected PBX-endpoint template, credential or approved address, observed source, challenge and response without publishing secrets.
SIP registration
DocumentedThe July 2024 manual describes each trunk as a SIP user, supplies an endpoint username and password, and gives the service FQDN for registrar and proxy configuration. Static authentication is a separate mode. Confirm the current portal template, transport, resolver results, registration contact, expiry and response for the selected endpoint rather than copying example credentials or fixed addresses.
Inbound delivery and number format
DocumentedNFON number routing assigns incoming PSTN calls to a trunk set by longest matching prefix. The endpoint template controls international or national number presentation and whether the inbound URI uses an E.164 number or the endpoint identifier. Preserve the called number, current routing object, selected trunk set, actual Request-URI and PBX match.
Multi-PBX and PSTN backup behavior
DocumentedNFON documents Premium multi-PBX modes for Primary/Backup, Round Robin and Forking. It separately documents a PSTN backup service when PBX endpoints are unreachable. These controls have different triggers and destinations: preserve the selected mode, endpoint priority or eligibility, registration or fault state, number-routing backup and new-call result. The public documents do not establish active-call survival.
Number and regional availability
DocumentedNFON markets centralised European connectivity and says number blocks can be supplied where its carrier interconnects make them available. Country rules, local presence, emergency handling and portability remain number and order specific. Confirm dated inventory, eligibility, port validation, activation and the applicable country terms.
Audio codecs, RTP and DTMF
DocumentedThe technical manual lists G.711 A-law and mu-law voice codecs and RFC 2833 DTMF events. Those interface descriptions do not prove the selected payload, packetization, media address, bandwidth or decoded digits on a call. Preserve both SDP bodies and observed RTP in both directions before changing codec, NAT or firewall policy.
Fax and T.38
DocumentedThe manual lists T.38 with G.711 fallback for fax and separately describes an optional e-fax workflow whose availability is qualified in the same document. Do not collapse SIP fax, fallback audio and email-to-fax into one capability. Confirm current entitlement, direction, gateway, SDP negotiation and a permitted document result for the selected order.
Signaling, media and fraud controls
DocumentedNFON lists TLS 1.2 and SRTP, call blocking, fraud detection, optional static authentication and user-agent checks. Confirm feature entitlement, transport, certificate validation, SRTP profile and keying, allowed identities and downgrade behavior. A TLS connection, approved IP or fraud-control setting does not independently prove protected media or authorized caller identity.
Concurrent calls and call starts
DocumentedNFON ties simultaneous calls to booked voice channels; the current product page says the XL option permits 50 simultaneous calls, and current German pricing lists channel packages. Catalogue channels do not establish the selected order, calls-per-second rate, burst treatment, access bandwidth, PBX resources or measured load result. Preserve each quantity separately and test only under an approved plan.
Emergency calling
DocumentedThe manual attaches emergency routing to the country of the trunk, the address assigned to the number block and the caller number supplied by the PBX. It warns that nomadic use away from the entered address may route incorrectly. Confirm the applicable country service, complete location, callback identity, change process, power and access dependencies, accessible alternative and provider-approved validation procedure; never place an uncoordinated emergency call.
Pricing and contract
DocumentedNFON publishes German SIP-trunk channel, package, activation, minute and flat-rate catalogue figures and explicitly says prices differ by operating country. Do not transpose those figures to another region or account. Preserve a dated local quote, currency, channel package, feature set, activation, numbers, usage, taxes, support, term and cancellation treatment.
Administration, status and support
DocumentedNFON documents a SIP-trunk configuration portal, a public status page and a 24x7 service desk in its July 2024 SLA. Partner-served customers may need the partner to open incidents. Preserve the active support path, maintenance window, incident timestamps, ticket number, endpoint evidence and restoration result rather than treating a status page as proof of one account.
SMS and MMS
Not verifiedThe reviewed SIP Trunk Flexx product, technical, SLA and pricing pages do not establish SMS or MMS eligibility, sender registration, inbound routing, throughput, retention or price for a selected number. Qualify messaging as a separate product and regulatory workflow.
Choose credential registration or static authentication deliberately
The technical manual describes endpoint username and password authentication and an alternative static mode using a customer public address. Confirm which mode the selected endpoint template uses, then preserve the FQDN, resolver answers, transport, source and destination, registration contact, challenge, response and expiry without exposing secrets.
A successful DNS lookup or TCP path does not prove registration, and a registered credentialed endpoint does not prove that a static-authentication address is current. Apply the narrowest documented firewall policy and avoid pinning example IPs or opening all internet SIP and RTP.
Bind numbers to trunk sets before debugging the PBX route
NFON says inbound number routing uses longest matching prefix to select a trunk set. The PBX template then controls number and URI presentation. For one call, preserve the dialed number, matched portal prefix, selected trunk set, chosen endpoint, Request-URI, identity headers, response and first PBX route.
A number in the inventory is not necessarily assigned to the intended trunk set, and an INVITE at the edge is not proof of a PBX dialplan match. Correct the narrowest routing object after proving the first divergence.
Keep four continuity mechanisms separate
Premium multi-PBX service offers Primary/Backup, Round Robin and Forking. Primary/Backup uses endpoint priority and readiness; Round Robin selects the next enabled endpoint; Forking can send parallel INVITEs and connect the first answer. A separate backup service can route failed inbound calls to PSTN numbers.
Choose one intended mechanism for the symptom. Do not treat simultaneous ringing as failover, a round-robin sequence as health-aware load balancing, or a PSTN backup number as proof that a secondary PBX is eligible. The documents describe new-call handling, not established-dialog survival.
Validate identity, media, security and emergency data independently
Preserve the selected caller identity, number format, G.711 offer, RFC 2833 telephone events, media addresses and both RTP directions. Where TLS and SRTP are ordered, verify certificate validation, the negotiated SRTP profile and protected packets instead of inferring media security from a TLS socket.
Emergency routing is tied to country, assigned location and caller number. Confirm those records and an accessible alternative with the provider; never use an uncoordinated emergency call as an acceptance test.
Close capacity, commercial and operational gaps
Bind channel count, simultaneous calls, call attempts per second, bursts, access bandwidth, PBX and SBC resources and provider enforcement to separate evidence. The current page advertises an XL catalogue option of 50 simultaneous calls, but that does not establish the selected package or a measured result.
Preserve the local-country quote, activation, numbers, usage, taxes, support path and maintenance window. Build a bounded non-emergency matrix for registration or static trust, inbound and outbound routing, identity, audio, digits, optional protection, each ordered continuity mode, recovery and rollback.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- NFON SIP Trunk Flexx product page ↗
- NFON SIP Trunk Flexx technical manual v3.1 (DE) ↗
- NFON SIP Trunk Flexx SLA v1 (EN) ↗
- NFON pricing ↗
- NFON service status ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.