CERTAIN INCOMING CALLS TO LANDLINE MUTED

Incoming calls (from a specific Corporate supplier) to my landline are muted! Callers can hear me but I can't hear them! After several wasted hours to Telkom Fault Reporting on 10214, 10217 & *** proved fruitless and most frustrating!! I Either got cut off midway or referred back to same numbers previously called!! My head is exploding, as no resolution or coherent human is sorting the problem!!