If I understand correctly, setting

encryption=no

means that Asterisk will make outgoing calls without encryption, but will be happy to accept incoming calls regardless of whether the caller wants encryption or not

If encryption=yes, then Asterisk not only uses encryption for the outgoing calls but it will refuse to accept incoming calls unless they use encryption too

If I have

encryption=no dtlsenable=yes

the DTLS support works but Asterisk will no longer accept incoming calls using regular RTP/AVP. These messages appear in the console and the call is rejected with code 488:

[Jan 28 11:08:42] WARNING[24673][C-00000009]: chan_sip.c:10496
process_sdp: Processed DTLS [FALSE]
[Jan 28 11:08:42] WARNING[24673][C-00000009]: chan_sip.c:10529
process_sdp: We are requesting SRTP for audio, but they responded without it!

I realise not everybody would set encryption=no in this situation, I’m simply trying to make it work for all possible callers to the SIP5060.net test numbers at http://www.sip5060.net/test-calls