Systems and methods for monitoring voice and SMS communication exchanged over a UMTS network is described wherein a technique for quickly testing different Count-C values, until the correct value is ascertained is used. The Count-C parameter consists of a HyperFrame Number (HFN) and the system iteratively attempts to decrypt the packets using a series of candidate HFNs. For each of these candidate HFNs, and for each of the packets, the system produces a candidate Count-C by appending the sequence number (SN). The system then uses the candidate Count-C to calculate a candidate keystream block. Subsequently, the system uses the candidate keystream blocks to decrypt the packets, and then tests whether the packets were correctly decrypted. If the system deems the decryption successful, the system stops iterating through the candidate HFNs, and uses the candidate HFN that correctly decrypted the packets to decrypt subsequent packets in the communication session.