Not really. Silence is encoded with silence frames containing just a length of silence marker in most VoIP protocols. On the receiving end, silence frames are t

Not really. Silence is encoded with silence frames containing just a length of silence marker in most VoIP protocols. On the receiving end, silence frames are then filled with "comfort noise". If you pay attention and have a crappy phone you can hear this, because the comfort noise generator will produce noticably different "silence". Yes, one often does CBR, but even there, variable difficulty of compression often produces variable jitter, which can then be used to infer information about the plaintext stream. There are constant runtime CBR codecs, but one has to take care to use them.