Yealink T22p Quality Issue

[Bessie64]

We have around 7 Yealink T22p.

Phone quality is good when only one phone is being used.

If there are more than two phones are being used at the same time, one of them breaks up a lot but it breaks up from the other end(so the issue seems to be uploading lags from our side). It means that I can hear the other person well but this person cannot hear me well as my voice breaks up.

They are all connected to a Cisco POE switch (2960-S). We have NBN Internet 80Mb/30Mb. Myprepaidbalance

Summary,  1. One call at a time is okay  2. If two devices are making calls or being used at the same time, one of them breaks out a lot. The test was carried out when the network is free during the weekend.

There are T41p phones and these phones do not seem to have a problem. How can the quality issue be happening to only all of the T22p models?

The test was carried out when the network is free during the weekend.