I am assuming that your grandson is still very much alive, and that it was his mother who you were speaking to.
There are documented incidences of EVP happening over the phone.
There is some evidence that at least a few of the messages we record in EVP, may actually be put there by people who are still in the physical.
Because you were speaking to his mother at the time the psychic, love connection which always exists between the three of you, may have allowed his thoughts of affection towards his grandma to temporarily impose themselves upon your phone conversation.
This would not be therefore from the other side, but rather it is further evidence of a greater reality, of which the physical universe and we are only a small part.
Paranormal or unexplained by normal physical means, instead of ghostly in the sense that the person whom we believe is communicating with us has already left the physical plane.
You say that it sounded like your grandson (although in a muffled voice).
Did it also feel to you like your grandson?