Outbound Caller ID

Our billing system identifies the customer to bill for the outbound call via the RPID header.

if we setup a trunk for each customer how can we fix the RPID header regardless of what they try to override? the from header we use for the outbound caller ID

so basically we are using the RPID header as a type of account number which belongs to each customer.

Enable RPID and set the callerid to the required value, in the dialplan.