Based on my experience I think their leaving your home phone as their contact information means that they don't expect to be living in their current housing for a long period of time.
I wondered why H/XH hadn't submitted an address change form to the post office after he moved into his mother's place. He lived with his mother for 8 months before he moved into the house he eventually bought. About a month before H moved into his new house it became clear that H had originally thought he was going to buy OUR house. He didn't think I could swing the purchase price myself.
