This kind of problems, which can include disturbed audio, "robotic" voice, echoes or similar disturbs, it's tipically caused by low performance of the internet connection.
If you are using a 3G internet connection try to get a better signal reception; if you are using a WiFi internet connection try to reduce the network load by closing other applications performing bandwidth-intensive tasks, like audio/video streaming, file sharing and similar.
However if you keep finding this problem with every calls, also when your connection is optimal, it may be helpful send a report to the developers, who can verify the presence of any anomalies.
To do this, right after the end of a call with bad audio quality click on Report a problem. You can find this button on Messagenet Talk app, clicking on Options and then on Help & info.
