IPO, IPOSE, IP Office, IP Office Server Edition: J179 - Intermittently there's no audio on incoming call.

Call flow:
Service Provider > Exp System IP500v2 PRI Line > Primary > User > Unconditional Forward to Hunt Group > Users

IPOSE 11.1.3.2
J100 4.0.10.3.2

-As per BP, it has been an ongoing issue since last year.
-BP shared monitor traces. I reviewed them and found no obvious fault in the signaling.
-BP shared new monitor traces with sniff on Primary and Expansion system. I confirmed from the sniffs that the audio from the external party was sent from the Exp System to the Primary server and from the Primary server to the J100 phone. However, no RTP was received from the phone. Thus, the IPO Primary relayed no RTP stream to the Exp System either.
-It suggests that the issue is with J100 or network.
-I requested new set of traces with sniff from J179 phone. I confirmed that the phone received the RTP stream from the IPO but never sent an RTP stream to the IPO. Only when the J179 user put the call on hold and then unheld it did the phone send the RTP stream.
-It confirms issue is with J179 phone.