Random one-way audio

I have an Asterisk 1.8.4 connected via OOH323 with an Alcatel OmniPCX. There is no NAT in between and I have ~1000 successful calls daily (most of them from Alcatel to Asterisk). A part of them (1 - 3%) are one way audio, the agent connected on the asterisk box doesn’t hear anything on the call coming from the Alcatel.
Also no audio on the recording made by asterisk.

I have searched in the logs but nothing relevant, no pattern has been identified so far. Looking at the traffic in the router is not easy since the one way audio calls are random and most of the time between other calls and I can’t isolate the RTP traffic.

Everything else is ok, quality perfect, setup stable but still… I have to solve the one way issue.

Any advise is much appreciate.