15 minutes in other party hears a busy signal, log shows nothing

We have an issue whereby calls will last for fifteen minutes, then a busy signal is heard by the other member of the call(not the local PBX extension). The caller placing the call does not hear the busy signal. Once the busy signal is heard by the other party, the other caller will not be able to hear what we’re saying.

I’ve checked the logs, and what’s confusing is that it doesn’t show any hints or error codes. I included a log here at bpaste: https://bpaste.net/show/925ccbc14d7e

I did a preliminary search and I see lots of posts related to busy signals on incoming calls or outgoing calls. These posts are about a constant busy signal; not a busy signal that only shows up for one party of the call after an extended period of time, so I am somewhat confused as to what I can do here. I can place calls and receive calls with no issue whatsoever, it is only once the call reaches approximately 15 minutes that it stops working.

edit: removing the term busy signal from any of my searches brings a lot of relevant information. I am going to read through this more before; my apologies for making this post prematurely.