А вы сделайте полный дамп в файл и посмотрите через Wireshark. Есть VAD & CNG встроенные в кодек,
G.729 has been extended in Annex B (G.729b) which provides a silence compression method that enables a voice activity detection (VAD) module. It is used to detect voice activity in the signal. It also includes a discontinuous transmission (DTX) module which decides on updating the background noise parameters for non speech (noisy frames). It uses 2-byte Silence Insertion Descriptor (SID) frames transmitted to initiate comfort noise generation (CNG). If transmission is stopped, and the link goes quiet because of no speech, the receiving side might assume that the link has been cut. By inserting comfort noise, analog hiss is simulated digitally during silence to assure the receiver that the link is active and operational.
https://en.wikipedia.org/wiki/G.729
и как отдельные опции, включаются и выключаются на трубках с кодеками, которые не поддерживают встроенный CN.
Real-time Transport Protocol (RTP) payload
format for transporting comfort noise (CN). The CN payload type is
primarily for use with audio codecs that do not support comfort noise
as part of the codec itself such as ITU-T Recommendations G.711,
G.726, G.727, G.728, and G.722.
https://tools.ietf.org/html/rfc3389
Или просто посмотрите глазами sip debug, пакет INVITE + SDP и ответ на него, в области maudio & кодеков.