Support for Deutsche Telekom's CompanyFlex SIP trunks

Voxbi's cloud PBX now supports Deutsche Telekom's CompanyFlex SIP trunks, thanks to our Bring Your Own Carrier (BYOC) capability. German businesses can keep their existing CompanyFlex trunks and run them on Voxbi's phone system, keeping full control of their telecom setup while gaining advanced PBX features. Interoperability has been tested end to end for reliable, smooth operation.

A challenge worth tackling

In 2020, Deutsche Telekom launched a new SIP trunking product, CompanyFlex. Unlike their earlier solutions, it is built on a different set of specifications, which changes both the admin processes and the technical configuration.

When a customer asked us to connect CompanyFlex trunks, our team took it on. Supporting the new standard was not a matter of flipping a few settings. It meant rethinking how our system talks to Deutsche Telekom's network. Think of it as teaching your PBX a new dialect.

We worked through the official 32-page specification (1TR119), which defines everything needed for a compliant, reliable connection. That meant deep research, packet analysis in Wireshark, and code-level changes so every SIP message was formatted exactly the way CompanyFlex requires.

Key changes for Telekom trunk registration

To meet the requirements for registering Telekom trunks, we made several key changes:

• DNS resolution: the outbound proxy is not resolved with a simple DNS A record. A query first returns a NAPTR record, which leads to SRV records that define the transport protocol (and optionally TLS), and finally multiple A records for redundancy.

• Proxy identification: an outbound proxy is required for CompanyFlex to connect properly.

• Client URI: the client_uri needs a different layout, which we set explicitly using the SIP user.

• Contact settings: the contact_user format needed adjusting, so we adapted how it is retrieved from the database. The SIP contact address is now derived from either the authentication part or the full user value.

• Qualify: qualify_frequency and expiration timers are tuned to their requirements for timely registration checks.

• Security: the code enforces SDES-SRTP and mediasec, with security negotiation set to mediasec.

These changes took significant work and extensive testing to be fully compliant with CompanyFlex requirements.

Built to keep up with telecom standards

We keep aligning with evolving telecom standards so the platform stays reliable and ready for what is next. Want to bring your own external trunk to Voxbi? Talk to our team and we will set up your integration.