Revenue and Customs workers in the PCS union went on a “lightning strike” on Monday of last week.
The dispute is over the private companies Teleperformance and Sitel taking over call handling trials at contact centres in Bathgate and Lillyhall.
Some 20,000 workers in contact centres, customer operations and enquiry centres walked out for half an hour at 10am and again at 4.30pm.
Other workers left work half an hour early. A work to rule and a refusal to work overtime is already underway.
A further strike is set for 31 January—the final day for self-assessment returns.