3 beeps and call is ended

We are having problems with calls received from traditional POTS (Plain Old Telephone Service or regular landline) with analog phones calling our FreePBX 13 with SIP Trunks. We are aware that in those land lines, there are other analog phones and analog fax machines sharing the SAME land line.

We are recording the calls in FreePBX and we noticed that every time a 3 beep sound is heard, the call is immediately dropped. We have been searching for answers and apparently those 3 beeps mean CALL CANCEL.  We have no idea what / where those 3 beeps are generated.

Any idea if there is a way to avoid the call to be dropped from FreePBX side? (Like a parameter that could avoid those 3 beeps)