Stewart Glancy left menacing telephone messages for Yazmin Thompson from Addiewell prison in West Lothian.
He also sent a letter threatening her with violence, Livingston Sheriff Court was told.
The accused had been sent to jail in March last year for assaulting Ms Thompson.
Glancy, 30, pleaded guilty to sending grossly offensive messages and behaving in a threatening and abusive manner.
Sheriff Andrew Webster adjourned the case until May 8.