In order to preserve battery power, Windows does not allow applications to run in the background. The only way to make incoming calls work while Zoiper is running in the background is to configure both the server and the client to use PUSH notifications.
You need to contact your VoIP provider regarding the support of PUSH.
You can find more information here. (http://www.zoiper.com/en/tutorials/push-notifications)
If you can not use PUSH notifications you will need to follow these steps:
Run Zoiper → go to Settings → select Lockscreen settings → enable "Run under lock screen". You will need to keep Zoiper open in order to receive incoming calls.