ISP blocking 5060

Hi guys,

Here is a problem
I move my RasPBX Server (Asterisk for Raspberry Pi) to new apartment and ONLY one ISP in building which is blocking 5060
After few days fighting with Asterisk, I discovered that best (probably only) solution change port

edit -> sip_general_custom.conf
add
bindport=1234
and also change settings on router to forward 1234 port to asterisk

All phones (Gigaset) goes online, but they can't make call (busy sound) only recieving calls

What couses this problem? And what is best way to go around when ISP blocking 5060 port?

Thank you