Calling our users is meant to be easy - but not too easy

You must use mutual TLS authentication

To protect our users from phone SPAM, we insist that you must have a TLS certificate, and whenever a call comes from your domain to ours:

  • a TLS connection must be made (not UDP or TCP, which are also common for SIP)
  • your certificate must be presented to our server as a `client' certificate
  • the domain in your certificate must match the domain used in the SIP messages, as per RFC 5922
  • We are happy for you to use free or low cost certificates (e.g. from, Thawte SSL123, which are as low as $US10 from some bulk online resellers, or GoDaddy SSL which are also quite cheap.) - although our list of supported CAs is periodically reviewed.

Suggested software for contacting us with TLS

  • The easiest solution is to use repro from reSIProcate (v1.8 or later),
  • A significantly more powerful solution, requiring slightly more effort to configure, is Kamailio.
  • or even Asterisk. If you do want to use Asterisk, we recommend using Kamailio as a gateway for all your external SIP connectivity anyway, because the TLS support in Asterisk is incomplete.

Find our users with ENUM

You don't need to create any specific routing configuration to send calls for individual numbers to us. You just use ENUM.

Our ENUM domain is - configure your SIP router to search this domain each time one of your users dials a number.

Find our SIP servers with DNS SRV

The ENUM lookup will return dynamically generated SIP addresses. There is no guarantee which of our servers will be used by the callee. Furthermore, our servers may be changed from time to time.

To cope with this, you must query our DNS SRV record (and not just the A record) to connect to the correct server. There is no A record for

This is not so hard though: most software, such as repro and Kamailio, has a flag that you can set to enable DNS SRV based routing. It may not be enabled by default, but it is essential that you do enable this feature.

Codec support

Please see our page on Codec recommendations