The 380x has a routing table function. There are two default route exist in the routing table, one is for IP incoming call another is for FXO incoming call, the IP call will be routed to FXO and the Call from FXO side will be routed to IP side.
If the 380x got an incoming call from IP side and the line number is not its local SIP number. Then the 380x will forward this number to the FXO which is based on the default route, and the FXO will dial this number through PSTN automatically. User just need to dial the destination PSTN number, this is called one-stage dialing.
For more information about one-stage dialing, please to to:
If the 380x got an incoming call from IP side, and the line number is the FXOs local SIP number, then the FXO will answer this call and user will hear dial tone from PSTN side, then they should re-dial the destination PSTN number. User should dial number twice, so this is called two-stage dialing.