'Service address' means the location of the telecommunications equipment from which telecommunications services are originated or at which telecommunications services are received by a retail customer. If this location is not a defined location, as in the case of mobile phones, paging systems, maritime systems, and the like, 'service address' means the location of the retail customer's primary use of the telecommunications equipment or the billing address the customer gives to the service provider, provided that the billing address is within the licensed service area of the service provider. A sale of postpaid calling services is sourced to the origination point of the telecommunications signal as first identified by either (i) the seller's telecommunications system; or (ii) information received by the seller from its service provider, if the system used to transport such signals is not that of the seller."