Separate Voice software, carrier service and the SBC
SIP Link lets an organization retain its existing carrier and telephony routing while connecting that service to Google Voice through a certified session border controller. Google bills the Voice software subscription, the carrier continues to bill calling, and the SBC remains an independently selected and operated boundary. It is not a generic bring-any-trunk interface.
Record the eligible Workspace subscription, country, carrier contract, number inventory, exact certified SBC model and firmware, public address, domain, certificate and support owners. The public country list is a product boundary; it does not establish site serviceability or number eligibility.
Documented service facts
Reviewed 2026-10-09. Compare SIP trunk services →
SIP authentication
DocumentedGoogle documents mutual TLS between the certified SBC and SIP Link plus a unique X-Google-Pbx-Trunk-Secret-Key header. The SBC certificate common name must use its fully qualified domain name and a domain matching the Workspace domain; wildcard certificates are not supported. Keep certificate trust, private-key custody and trunk-secret rotation as separate controls.
SIP registration
Not verifiedThe reviewed documentation establishes a mutually authenticated trunk and secret header, but it does not establish a PBX-style user SIP REGISTER workflow. Confirm the selected carrier and certified-SBC integration, then preserve trunk status, SIP OPTIONS where used and an actual call independently; do not translate a certificate handshake into registration success.
Inbound delivery
DocumentedA carrier-owned number must be added to the SIP Link trunk, verified and assigned to a user, auto attendant or ring group. Google's troubleshooting guide says an unassigned number can produce 404 or 604 and requires a P-Asserted-Identity in E.164 form. Carrier activation, Google assignment, identity acceptance and final destination remain separate checkpoints.
Routing and failover
Not verifiedGoogle lets an administrator add multiple SBCs and identify the active hostname for a high-availability pair. The reviewed pages do not establish generic load sharing, failure timers, access diversity, carrier rerouting, DNS behavior or survival of established calls. Record the certified design, active and standby roles, failure trigger, observable route and restoration sequence.
Number and regional availability
DocumentedGoogle's current public page lists SIP Link availability in 34 countries and separates countries using Voice Standard or Premier from SIP-only countries using SIP Link Standard or Premier. This is a dated product-country list, not proof that the existing carrier serves a requested site, that a number is eligible or that a subscription can be ordered through the chosen channel.
Audio codecs and DTMF
DocumentedGoogle documents PCMU, PCMA, Opus and G.722 plus RTP telephone events. It also says the media offer must use the configured DTLS or SDES method rather than both. Preserve the actual SDP, selected codec, packetization, media endpoints and decoded digits; the supported list does not prove one call's negotiation or quality.
T.38 fax
Not verifiedThe reviewed requirements, troubleshooting and subscription pages do not establish T.38, G.711 pass-through, switchover, redundancy, certified-SBC behavior or completed pages for this workflow. Treat fax, alarms and other non-voice devices separately and require current carrier, SBC and application evidence.
Signaling and media encryption
DocumentedSIP signaling to Google uses TLS 1.2 or later with mutual authentication. Media uses either DTLS or SDES, and Google says unencrypted media offers are terminated. This documents the Google-to-SBC leg only; record the carrier-facing signaling and media profile separately and do not describe the complete path as end-to-end encrypted.
Concurrent calls and call starts
Not verifiedGoogle gives a planning estimate of about 134 kbit/s for one SIP Link-to-user call: 84 kbit/s on a G.711 SIP leg plus 50 kbit/s on the Opus client leg, or about 13.4 Mbit/s for 100 simultaneous calls. That is not a service concurrency or call-start limit. Size subscription, SBC, carrier, access, firewall and PBX constraints separately.
Emergency calling
DocumentedGoogle's subscription comparison marks 911 emergency support for SIP Link plans, and the administrator FAQ says the physical location entered for a user is sent as the emergency service address. Complete the applicable country number, carrier route, user location, callback identity, notifications, outage behavior and approved validation; a saved address is not proof of dispatch readiness.
Pricing model
DocumentedGoogle's current comparison publishes SIP Link Standard at USD 6 per user per month and Premier at USD 8 per user per month. These are dated public software-subscription prices, not a quote. The existing carrier continues to bill calling, while SBC, number, access, tax, currency, support and contract terms remain separate.
Administration and support
DocumentedWorkspace administrators create the SIP trunk, add certified SBCs and numbers, choose one media-encryption method and assign destinations. Google's troubleshooting workflow depends on SBC and firewall packet captures and logs. Assign owners for Workspace, carrier, number, SBC, certificate, secret, firewall, emergency data, monitoring, escalation and rollback.
SMS and MMS
DocumentedGoogle's subscription comparison says text messaging is not supported for SIP Link numbers. Do not infer SMS or MMS from voice-number presence. If messaging is required, qualify a separate current product, number eligibility, sender registration, API, throughput, webhook security, retention and commercial record.
Build the protected trunk from exact identities
Google documents mutual TLS for signaling, a public certificate whose common name matches the SBC FQDN, and a domain matching the Workspace domain. A unique trunk-secret header adds a second identity control. Media must use the encryption method configured for the trunk—DTLS or SDES—and unencrypted offers are rejected.
Keep Google-facing and carrier-facing security evidence separate. Prove the certificate chain, hostname, secret placement, firewall path, SIP target and selected media profile without treating a successful TLS handshake as a completed call.
Prove number presence, assignment and routing separately
A SIP Link number is added to the trunk, verified and then assigned to a user, auto attendant or ring group. Google's troubleshooting guide distinguishes missing assignments, malformed E.164 identity, wrong request-URI host and an incorrect trunk secret. Each can stop the same call at a different boundary.
Preserve carrier activation, Workspace number state, assigned destination, P-Asserted-Identity, request URI, sanitized SIP response, PBX decision and user outcome. A number appearing in the console does not prove that the carrier delivers it or that the final destination answers.
Turn bandwidth and multiple SBCs into testable limits
Google supplies a bandwidth estimate of about 134 kbit/s for a G.711 SIP leg connected to an Opus user leg, but that is a planning input rather than a published service capacity. Concurrent-call entitlement, call starts, SBC resources, carrier channels, access capacity and firewall state are separate constraints.
Multiple SBCs can be configured, including an identified active hostname for an HA pair. The public requirements do not establish a universal failover timer, diversity design or active-call survival. Preserve the accepted topology and observe failure and restoration with approved tests.
Bind subscription, emergency and messaging claims to the order
Google's current comparison lists SIP Link Standard and Premier software prices and says SMS and MMS are unavailable on SIP Link numbers. The supported-country page distinguishes the subscriptions available in Google calling-plan countries and SIP-only countries. Preserve the dated order and carrier bill rather than extending public list prices to taxes, currency, SBCs, numbers or support.
Emergency support still depends on the applicable country, carrier route and accurate user service address. Complete callback, dispatchable-location, notification, outage and approved validation records; do not infer readiness from subscription eligibility.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- Google Voice SIP Link requirements ↗
- Manage SIP Link numbers ↗
- Troubleshoot SIP Link ↗
- Google Voice and SIP Link supported countries ↗
- Compare Voice subscriptions ↗
- Voice FAQ for administrators ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.