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RTP analysis jump in forward delta time

Hi, I am trying to find the cause for strange audio problems that we see sporadically at our Alcactel PBX. It sometimes happens that suddenly the external phone partner can not hear the audio from our PBX anymore. I captured some of the offending RTP streams and analyzed them with Wireshark. There I realized that the audio problem correlates with a jump in forward delta time - the forward delta time suddenly changes from 30ms to 40ms.

During the period where the forward delta time is 40ms the external partner can not hear the audio, although when I play the audio track in Wireshark the audio is played just fine. When the forward delta time returns to 30ms the external partner can hear the internal partner again.

Here a screenshot of the Wireshark graph:

I read in other forum posts that a change in delta time can happen when silence filtering happens but in the RTP stream I see no markers for that.

Does anybody have an idea what is going on here? Any help or hint is appreciated!