I seriously doubt that the Petrie house would have the same phone number as it did back when Laura and Rob lived there in their younger days. I mean, Richie bought the house several years after Rob and Laura had sold it, plus phone number don't remain as part of the house!
It's been awhile since I've seen the movie but could Rob and Laura have sold the house to Richie? You make it sound like there was a buyer in between Rob/Laura selling and Richie buying. If that's the case then it is strange about the number staying the same. But like I said, I haven't seen it in years and it sounds like you may have seen it recently.
To me it sounded as though there had been a buyer in between Rob & Laura selling and Richie buying. I saw the movie when it came out, but that's something that has always stood out for me.