Could you provide the reason for stopmonitor error as app could not make Outbound call and giving the error

Exception caught in StopMoniter
[
{
"ClassName": "System.ArgumentException",
"Message": "MonitorId () is not a valid integer."

We would need a lot more context than you provided to even speculate what the root issue may be (i.e. what triggered stopMonitor()... the monitorID is a manditory parameter and it is missing apparently. However I infer that you dont feel your code called StopMonitor(), if that is the case then what did, and why become the questions.

That level of troubleshooting support is available to Support Enabled and higher level members if they can provide full logs of the use case causing the error via Technical Support tickets.