Sometimes the customer will find that they have a handset that is incompatible with a MyNetFone device.
The cause of the incompatibility is wide and varied, however there is something you can try to troubleshoot this issue.
IF the Analogue telephone adapter/VOIP router has any setting pertaining to Gain, you can alter these settings to match those of the handset (generally labelled at the base of the handset/receiver)
Alternatively, you can alter the input/output gain by + or -1 to see if that fixes your problem.
If you are still having any issues, please try an alternative handset.