It is not recommended to use the same account on more than one app at the same time. You will need a second account associated to that extension to be able to receive calls on both devices.
Usually, the calls will be sent to the device that last registered the account. If the device unregisters the account, the calls will be sent to the last device that sent a re-registration request.
In your case, you need to unregister the account from the mobile device, then turn off zoiper on the mobile device, unregister the account on your desktop pc and then register it again.