Prisoner Martin Tate says he is not allowed to make calls in Welsh
A prisoner serving a 16-month sentence claims he is not allowed to speak Welsh over the phone to his son.
Martin Tate, from Caernarfon, was jailed last year after admitting aggravated vehicle taking and assault.
His constituency MP, Hywel Williams, has taken up the case with the prison management at HMP Holme House in Stockton-on-Tees on Teeside.
The Prison Service said a translation service was available to all prisoners if they gave 48 hours' notice.
Mr Tate, 25, has said the prison cuts off phone calls to his family if he speaks in Welsh.
His mother, Linda Jones, said her son was previously at Walton and Altcourse prisons where he had not experienced any problems speaking Welsh to her.
She also said she was not aware of a rule that required 48 hours' notice of a phone call to allow for a translator to be able to listen to conversations where it was deemed necessary for security reasons.
"I speak to my son three times a week. Giving 48 hours' notice each time would be ridiculous," she told BBC Wales.
"He should be in prison for what he's done. He should be punished, but punish him not punish us or his baby.
"We haven't done anything and why do we have to talk English because he's done wrong?"
A Prison Service spokesman said they did not comment on individual prisoners, but some prisoners had a list of approved numbers which had to be tested by staff.
"Prisoners who have to have their phone calls monitored for security reasons are able to make phone calls in any foreign language providing they give 48 hours' notice for the interpretation service," he said.
The Welsh Language Board said it was "unacceptable".
Arfon MP Hywel Williams said the case had prompted calls for a prison in north Wales.
"This sort of thing wouldn't have happened if he had been in a jail say in Caernarfon or somewhere else," he said.
"It's a complete injustice as far as I can see."