If I understand correctly - all lines register OK, inbound calls on PSTN OK, problem with outbound calls on PSTN. Lots of possibilities. You need to provide information like PSTN Gateway Info, copy of logs, device being used for extension, that sort of stuff..
Footbridge in use linksys spa 3102 IP 192.168.1.20
extensions 100-101-102 registered ok
line 10000 registered ok
Problem for calls of exit in fxo
state of the servant 3cx
10:14: 53,428 CallConf:: Rejected [CM103005] Call (38) is rejected: Not Found
10:14: 53,428 StratInOut:: onCancel [CM104008] Call (38): Call from Ext.101 to 955999727 terminated; cause: 404 Not Found; from IP: 192.168.1.20
10:14: 53,147 CallConf:: onIncoming [CM103002] Call (38): Incoming call from 101 (Ext.101) to sip: 955999727@ 192.168.1.10