There are two types of call forwarding: server-side and client side.
The server-side forwarding is set on the server(PBX), while the client side forwarding is set inside Zoiper.
Those can be useful in different scenarios.
For example the server-side forwarding should be chosen when you need your calls to be forwarded while your Zoiper is not registered and your computer is not reachable. Then the server can send the calls to your mobile phone directly.
If you need to set forwarding locally into Zoiper(it will not work when Zoiper is not online), you can do so from Zoiper -> Settings -> Calls -> enable "Enable incoming calls handling".
Then you need to choose if you want instant forwarding or delayed one.
The instant forwarding happens immediately when a call comes, while the delayed one happens with some delay between 1-600 seconds. The default value is 30 seconds.
Choose whatever you need and fill the target number where your calls should be forwarded under "Forward target:".
Keep in mind that the configured forwarding will be active only until you restart Zoiper. If you want to settings to be persistant you need to also choose "Keep settings after restart".
If you see errors on incoming calls and they are not forwarded:
This might happen if forwarding is not allowed for your SIP account. You would need to clarify this with your VoIP service provider.
You should also clarify with them if they support: "SIP 302 Moved temporary".