The only way is called Call Classification. You can configure it on the CallManager Administration page at the gateway which connects the internal and the external location. It has two states (OffNet, OnNet). The states indicate that the calls coming from that gateway are offnet or onnet for the local network. So, if somebody from outside, calls your phone via that gateway you configured OffNet, you will hear a different ringing type than in the case of OnNet calls. Different ring type means the sequence and the length of the ringing, not different sound.