There are 3 different places where you can setup technical prefix:

1) For the inbound traffic on the authentication rule of every account:

When you add technical prefix on account's authentication rule it influence on a call routing and system would demand this prefix in a tariff rate list that account use and then in the destination sets of routing that account use.

The following translation rule will add 53872# prefix:

  • s/^/53872#/

See more rules in the following documentation:

Understanding Number Translation

2) In account properties for calls from the registered on account device.

3) In connection properties for outgoing calls to the vendor. 

In such particular case system adds prefix for CLI or CLD number before the actual send of SIP request to the Vendor.