If your call has failed, here's what you should check first:
You probably have calls enabled, but the person you're trying to call may not. Double-check with them, and learn more about how to enable calls here.
Is the person you're trying to call in your contact list, and are you on their contact list? To call someone on Session, you must have sent them a message request, and they must have accepted this request. If you haven't already done so, send them a message request. Once they've accepted, you'll both be in each other's contact lists and able to make a call.
If you think your calls are failing for a different reason, please contact the Session support team using the link below so that they can assist you.