TCP or UDP all allowed but this is not the case because 3CX uses UDP.
This topic ( http://www.3cx.com/forums/3cx-only-tcp-voip-provider-40294.html ) is more convenient for my situation but as it was writen in that topic, there was no solution before.
What about now? is there any solution?
By the way, far end SBC configured to listen the port and TCP communication works with softphones directly.
Is there any option for SIP Trunks like server mode or client mode?
Thanks