Trunk/Line getting 'Stuck'

Hi, I’m having a bit of an issue, the bad news is, I haven’t being able to identify the cause, but quite often I get call stuck, I think it happens when we transfer a call to an external number but to be honest I’m not sure, the problem is not so big since I can resquest hangups, the bad part of that is that I need to be told we can’t receive calls or notice it myself.

I know I’m being kind of vage, but truth is I don’t really know how to pinpoint the nature of the issue.