We cannot control how much time it takes your operator to effectively forward the call to your mobile phone. This depends from operator to operator and may even depend from day to day. If you set your forward duration to 15 seconds and your operator needs 10 seconds to effectively forward the call, then you'll hear it ringing for only 5 seconds.
To resolve: set the duration longer in the Incoming Calls screen.

Warning! If you set the duration (in de Incoming Calls screen) too long, your caller might end up on your mobile phone's voicemail (not the Flexphon voicemail). Ask your operator how to disable their voicemail or increase the time after which your operator will switch to their voicemail.