SOLVED by changing Asterisk version (upgrade) for this issue: https://community.asterisk.org/t/asterisk-15-chan-sip-tls-srtp-no-audio-no-rtp-packets-between-ip-phones/95043

Asterisk 15 chan_sip TLS SRTP - no audio no RTP packets between IP Phones

I just wanted to let everyone know here in the Asterisk community that I have managed solving this issue by upgrading the Asterisk from 15.4.1 to 16.26.1 and it works good now all the calls are Encrypted TLS and SRTP, with the same configurations that I have provided here for Asterisk 15, so it seems maybe it was Asterisk version issue (any kind of bug) while 15 was not LTS version as it was Asterisk 16.

tested OK