The National Association of Schoolmasters Union of Women Teachers is giving the plastic gadget to anyone who joins before April 1. The holders flash a red light when the phone rings.
Union officials decided gadgets, emblazoned with the NASUWT logo would be suitable enticements after noticing they were popular at teaching conferences.
The union says the holders, which can be bought in bulk for less than pound;1 each, will be useful to teachers when they need their mobiles to be silent but still want to notice incoming calls.
Chris Keates, deputy general secretary of the NASUWT, said the holders had been praised by members who had hearing difficulties because they alerted them to calls.
The offer, advertised in today's TES, did not impress the rival National Union of Teachers. Arthur Jarman, head of membership, said: "We prefer to spend members' money campaigning for their interests."