Oh, come on. It is entirely possible that the phone company got the lines fixed by the time he orders the pizza. The phone company probably got it done faster than they expected.
I think the phone company did repair it didnt they(aint seen the film in ages but i remember the phone people doing something) and they must have fixed them at some point because the police phone the house dont they to check that Kevin was ok?
As the family is rushing to get in the vans to the airport, one of the electrical workers tell's the mom that the power is back on, but it will take a couple of days for the phones to be fixed. So, you would think they wouldn't be fixed for a while, which would also explain why mom never called Kevin at home.
And, shouldn't the neighbor phones be out? We later hear a message on the neighbor's answering machine while they are being robbed.
I work for the phone company... we can tell someone that they will be out of service for over 24 hours and actually get it working in a matter of two hours or less...
If the phone lines were back then would Kevin's parents not just try and phone the house? Surely she was listening when the guy was talking about the phone lines.
The reason he is able to order pizza but his family can't call him is that the main trunk line was severed, but calls within the same exchange would still connect.
http://www.independent.co.uk/arts-entertainment/films/news/home-alone-s-major-plot-hole-has-been-explained-a6676721.html