Why can there be an echo when calling from IPPBX to a cell?

Question

Asterisk

Hello.

The IP device / software calls the subscriber.
The call goes through Asterisk to the Grandstream IPPBX and from there is forwarded to the mobile device via a CO line (PSTN).

There is an echo on the side of the mobile device. The question is, how can this be tracked / defeated?

Sincerely,

Answer 1

through a city line (PSTN).

The first place to look for echo formation.
The second place is the bad channels of the ip itself (unstable ping\narrow channel\packet loss). This is especially possible if mobile Internet is used somewhere.
Well, then look directly at the codecs.