Hi.
Please tell how you isp block you ip, do you think srtp will help you?
SRTP used only for voice data (RTP), voip have also signaling protocol. so you need also TLS setup for sip/sdp.
I can setup it for you, but I can't guarantee that it will help.
I think it is possible use TLS/SRTP only for sip calls, not iax.
What you mean SYNC?