Sorry for the late response.
Check if your circuit has an echo suppressor – very common for internation circuits.
For early deployments of GSM / PSTN, echo was an issue. Later solutions used Echo Cancellers (EC) in pool. The signalling and the selection of a route to the PSTN determined if an EC resource was required. Later solutions could address NEXT, FEXT and Mobile Cross Talk (XT).
VBR/ Wallis Dudhnath