Since the coalition government took office in 2006, it has become permissible for employers to intercept work calls and other conversations that employees make during working hours. This is terribly offensive.
If it is done to check that the employee is constantly productive, then this is against medical wisdom. The only prerequisite for humans to cope with stress is that there is time for recovery. Then it is stressful to know that the employer is constantly checking you and does not trust you. Back to freedom under responsibility!