I'm having an issue with transfering SIP calls from one UA to another with Mikrotik HAP AC router. There is no problem with blind transfers. We only have problems with attended transfers. Terminal A established a session with terminal B. Terminal B puts terminal A on hold. Terminal B calls terminal C to announce the transfer and puts terminal C on hold. Terminal B trasnfers terminal C to terminal A. Terminal A replaces the session between B and C, but INVITEs don't progress. Terminal C does not receive the INVITEs.
Any ideas?
Rgds
Maribel

If A,B,C are all at the private side of the NAT done by the Mikrotik, and the PBX or public exchange is at the public side, and the PBX is able to deal with client side NAT on its own, switching off the SIP ALG on Mikroik may help (/ip firewall service-port disable sip followed by reboot to be sure it was actually done). If this doesn't help, I'd have to see the complete SIP exchange from all phones and the WAN to maybe suggest something (chances are low).