Identifying an incoming call as from a mobile device of an enterprise communication system. Creating a first list relating mobile devices of the enterprise communication system with Public Land Mobile Network (PLMN) number of the device. Creating a hashmap comprising keys. Each key comprising the last n digits assigned as a PLMN number to at least one mobile device of the enterprise communication system. The hashmap further comprising a plurality of buckets. Each bucket corresponding to a key, and comprising a second list identifying each enterprise-associated mobile device having a PLMN number ending with the key. Receiving an Automatic Number Identification (ANI) number of an call incoming to the enterprise communication system. Retrieving at least one bucket corresponding to a key comprising the last n digits of the ANI number. Searching the retrieved bucket for the unique identifier associated with the full PLMN number.