Zoiper is a multi-platform softphone which offers contact integration, conferencing, encryption and more.
The Community version is free but has limitations on some features, such as call transferring.
See Zoiper website for more information on Zoiper.
If you are using Zoiper on Android then please see the Zoiper for Android Setup Guide.
For more in-depth information on setting up Zoiper please consult the help guides at the Zoiper Support Questions site.
If you can not register, check over your username and password, and that the SIP settings have been entered correctly .
If you are copying and pasting the password please ensure you do not accidentally copy a space at the start or end.
You may also need to make some changes to the advanced settings.
You can access this by clicking Settings → Advanced
Under Global STUN ensure Enable STUN is disabled (not checked).
You should also ensure that only g.711 a-law codec is selected for your account.
You may also wish to set the QoS values if you have a router which supports QoS: