To deactivate the link between the car and the Volvo On Call app, you need to be logged into the app. If you do not have access to the app, contact your Volvo dealer and explain that you want to sell your car.
You can terminate your ownership under Connected cars in the 
tab. Select the car in question and terminate the ownership by following the instructions in the app.
User history, driving journal and other user accounts will be deleted once you have terminated the ownership. Contact your Volvo dealer if you want to delete your contact details from the dealer's system.
If you have the opportunity, notify the buyer that they should contact an authorised Volvo dealer in order to reactivate Volvo On Call and provide their contact details. Volvo On Call will be active until the subscription expires.