Here is some more information about my environment: It's a home and home office environment with one Cisco 7975G, one Yealink T22P and a Yealink W60P with two handsets. Secure SIP is working well for the Yealink W60P, but while the Yealink T22P registers just fine, there is no audio during calls. While the phones sit on a different subnet, they can easily reach the 3CX subnet using an IPSec VPN. So, it shouldn't be a NAT issue. All Yealink phones are provisioned by 3CX. Once provisioned, I have uploaded the certificate, changed the transport protocol to TLS and enabled secure RTP. I have left all ports unchanged. Is anybody using a similar setup successfully? Any feedback is appreciated!