In order to fix the issue, I recommend that you disable the call forwarding option on your device. Press the Call button, tap the Menu key and open the Settings list. Look for the Call Forwarding option, tap Voice Call, select Always Forward and disable it. Alternatively, you can try to remove your SIM card. If it doesn't help, I recommend contacting the developers. You can get in touch with them using
firstname.lastname@example.org email address.