A system and method for determining a tariff for a real-time wireless service such as a prepaid service (PPS) for a wireless telephone call. A Local Number Portability Point (LNPP) provides an interface between service scripts in a Service Control Point (SCP) or application server, and an LNP database. When an Initial Detection Point (IDP) message is received by the service scripts, a query is sent through the LNPP to the LNP database to determine whether the call is made to or from a ported telephone number. If so, a routing number is returned. The scripts determine whether the routing number points to the network operator's own network or another, and notify a Service Data Point (SDP). If the routing number points to the operator's own network, the SDP applies a first tariff to the call. If the routing number points to another network, the SDP applies a second tariff.