Workers who answer 999 calls claim their toilet breaks are being limited.
Staff say they were informed of a new toilet break policy just hours after they announced their intention to ballot for industrial action.
They're demanding to be paid the living wage of 11.50 an hour.
The Communications Workers Union, which represents the staff, says the new policy allows a maximum of 7 minutes for a single toilet break - and if the emergency call operator needs more time, they have to seek specific permission from line management.
They are allowed a total of 19 minutes of toilet breaks during a 12 hour shift.
