If the left side is the PBX then these calls are outbound calls, this you can determine by the fact that the first INVITE is sent by the PBX. Now if they are incoming calls that just got routed via the PBX out to the Provider again that's a different story.
In any case, the "415 Unsupported Media Type" message would be sent by the PBX due to the "MESSAGE" SIP message sent from the Provider, but this should not cause a call drop and from the looks of it it indeed isn't the reason. First off you have it in both call scenarios however you got different behavior between these calls and also, for the second call, the drop occurs some time after the message which further suggests it's unrelated.
All I can say for sure from the screenshot is that it is the Provider ending the call. This you can tell from who sent the "BYE" message:
View attachment 28766
I suggest you do find a way to reach out to them as only they can shed some light on the matter since it is they that end the call.