An incoming call hangs up on the gateway (IP telephony). What could be the problem?

Question

Asterisk

Good day.
I must say right away that I am in the ranks of a beginner (it sounded scary) * rofl *
The task is this: the "External" subscriber calls the enterprise, dials, if you do not have time to pick up the phone - the call hangs up (the subscriber hung up, but my phone still rings)
On gateway I see that as if the line is busy. At the operator's PBX they see that the line is busy.
If you pick up the phone there will be just silence. you put the phone back, the call is dropped and everything works fine again.
If you have time to pick up the phone, then everything is perfectly audible, everything works.
already left an application for a telephony operator. for its part, I decided to "dig" what could be wrong?

Answer 1

Without the attached logs, we can safely say that the problem is in retrograde Mercury.

Answer 2

Communication between provider automatic telephone exchange and the gateway - analog? Then install chippers or try to set up clearback frequencies on the gateway.