Hold Disconnecting

[Stevev]

Customer is using a combo of T4x phones in a 3 location environment. They are having an issue when a call comes into a phone and the call is put on hold, the call gets put on hold but disconnects after 15 seconds and you cant retrieve the call before it disconnects by pressing the flashing line key either. This works fine at the Main site where the internet is centralized, but the two other locations, which connect back to the Main site via MPLS, are the ones having the issue. So this seems to be an issue with the MPLS between the sites since it is only happening at those locations and the SIP traffic traverses over the MPLS to go out the internet at the main location. Placing outgoing calls on Hold works fine and all other calling functionality appears to be working fine as well. Taking the phones offsite will work. Network admin has verified that they are not blocking anything and that they do not have any SIP ALG settings enabled on the routers (ASA5510) between sites. ISP says they dont have any SIP ALG settings on their equipment(Adtran Total Access 900es and ASR9000 core).
We looked at the SIP stack and we never receive an ACK for the 200 OK (for INVITE) from the phone side that placed the call on hold. (Attaching Bad call and good call with highlighted response we are not getting from phone side.)
So at this point I am just reaching out to see if anyone has any ideas?
Is there something specific about putting a YeaLink phone on hold that would help us point to what is going on?