Audio quality is very poor with live connection

When we try to talk to someone that has rung the doorbell, the audio quality is so poor that we can't understand each other. There are gaps in the audio. All the tests on the app indicate everything is fine: transformer voltage, RSSI, internet connection speed etc. We get 20+mbps download and 2.5mbps upload. The doorbell is about 2 metres from the wifi access point. Any ideas what I can do to resolve this?