I am experiencing a strange issue with an end user using MiCollab client 7.3. When calling an internal extension, they can hear the other side but the other side cannot hear them. However, when calling an external number, e.g. mobile number, both sides can hear each other. I tested this by calling the internal number of reception where there is one-way audio, but when dialing the external number of reception both sides can hear each other. What could be the cause of this issue?