RTP Timestamp problem?

I’ve noticed recently that asterix seems to occasionally have a jump in the timestamp… Is this supposed to represent silence? It seems weird that it only increments by small amounts sometimes, i.e. 160, or 320, while in the middle of a talk spurt… If it truly is silence, then the marker bit should be sent at the end of the silence period, but it is not being sent.

Anyone have any ideas?