Busy circuits error
I have two tg1600 models with 15 sim cards on each one. I configured them for a call center with grandstream ucm6202. I matched every trunk on the gateway with a grandstream ip phone to make outbound calls only. Now my problem is some calls every now and then give the error "all circuits are busy please try again later" wich comes up on my phone as a missed call with a slight ringtone in the beggining. I dunno 8f it's a signal issue or a config issue i tried to find some sort of a pattern to detect what's the problem exactly but it's all random and i got confused. i even changed the sim cards but the issue remains if anyone got any ideas I'd really appreciate the help now my call center is suspended until i find the fix for this issue.