Got 2 SIP gateways (ZultysMX250). Sip trunk between them, they are in separate locations, different subnets 192.168.XX and 192.168.YY connected by IPSec VPN.
Some phones work perfectly. But mine... (and I'm the admin.. grrr) is a bit older, and the guys at the other office can all call me OK, but if I call them, they can't hear me, but I can hear them.
Firewall rules on the VPN are identical to the other phones - all the Aastra 53is work, my Zultys 4x4 doesn't.. some of the 2x2s are strangely intermittent... polycom soundstation IP6000 suffers the same problem as well.
Don't suppose anyone can give me an answer, per se - but where should I look next? Could it be a codec issue?
Edit: now been narrowed down to just a particular phone brand (the original Zultys own brand).