I solved the issue today:
Here is how to setup SIP-over-TLS SRTP for voip.ms:
- Install Jami from F-droid
- Create a SIP account following this post
- Follow the Jami wiki on voip.ms for the basic configuration.
- Go to the security options of the SIP account
- Activate: Encrypt media streams (SRTP)
- *** Activate: Enable SDES as key exchange protocol ***
- Activate: Use TLS Transport