Secure, digital, packet-switched, voice telephony calls are wirelessly transmitted in an efficient manner that reduces air interface bandwidth consumption by encrypting only vocoder data frames containing encoded speech, and not encrypting vocoder data frames that do not contain speech, such as those containing silence parameters. The collective reduced bandwidth consumption across a large number of voice telephony calls may allow for the admission of one or more new calls during times of voice telephony congestion. Not encrypting the silence parameters does not compromise call privacy or security, since the silence data frames do not carry any speech. The classification of encoded data frames as containing speech or not may be performed in a variety of ways.