Your situation is quite common — hasty resignations and dismissals do happen, writes Peter Done, managing director of Peninsula. Tribunals and case law recognise that, on occasion, individuals may lose their temper. To resolve the situation, whoever is in the wrong should apologise. The employee should be allowed to return if he is at fault and be asked to return if the employer is at fault. Refusing to allow him to return would normally be seen as unfair dismissal.
You cannot take his telephone call to be a resignation. In this situation the employee has returned to work and you need to look at it in this context. An employment tribunal would consider whether refusal to allow the individual to return to work was reasonable.