The recipient of the texts thought they were prank messages from friends.
A woman was horrified to receive a text message from her grandmother's phone number - three years after her death.
When Lesley Emerson died in 2011 some of her favourite things were buried with her, including her mobile phone.
Sheri Emerson, of South Shields, said she found comfort in texting her but was stunned to get a reply, saying: "I'm watching over you."
It emerged her grandmother's number had been given to another user who replied, thinking friends were playing pranks.
The network in question, O2, said it had spoken to the family and apologised for any distress caused.
Ms Emerson said that following her grandmother's death, rather than visit her grave at Harton Cemetery in South Shields, she would text small, personal messages as a "way of being close to her".
"I know she's not alive, but it's still going to her," she said.