One solution for prepaid customers who want to make calls while abroad (roaming) is by using the USSD Callback (UCB) service. With this service, the customer (A number) sends a command to the home network. The network will then automatically call the customer back and connect the call to the destination number (B number). Since this service uses USSD, Tri customers who are roaming only need to dial the following code: *133*# After dialing the code, the system will call you back and connect your call to the intended number.