I have a Cisco CP-7841 working using UDP/RTP to my FreePBX 22.214.171.124/Asterisk 17.4.0 server.
I want to use TLS/SRTP.
I have reviewed this list https://www.cisco.com/c/dam/en/us/td/docs/voice_ip_comm/cuipph/all_models/ca-list/CA-Trust-List.pdf and installed a certificate from Godaddy on my FreePBX server.
I have configured my PJSIP channel settings to use my Godaddy cert, tls - 0.0.0.0 = yes, SSL method = default. I have configured an extension transport = 0.0.0.0-tls, media encryption = SRTP via in-SDP, Allow Non-Encrypted Media = No.
I configured my CP-7841 SIP Transport = TLS, SIP port = 5061
My Zoiper Softphone connects using TLS/SRTP.
My Cisco CP-7841 never registers.