Teacher Support Cymru, which runs the confidential 24-hour line, also revealed that a third of calls were about working conditions, particularly pay, workload and school management.
The rest of the calls (13 per cent) were about health and well-being. The charity cited the example of Mary, a maths teacher, who felt she had been undermined since receiving a low grade in an inspection.
Speaking anonymously, she said she had become increasingly monitored and undermined. "I dread going to work," she added.
Counsellors at Teacher Support Cymru are helping her decide whether to confront her head, find another post or take legal action.
Telephone counselling is on the increase for teachers, according to chief executive Patrick Nash. The charity will launch a bilingual counselling service later this month, providing information, factsheets and one-to-one support via email.