I REALLY hope Sims don't all have mobile phones by default. It makes it pointless for Sims to have house phones.
The only time house phones are used in TS3 is to tell you someone is about to die!
In TS3 Seasons the chance of a townie getting an umbrella or parasol is set at 80% so not ALL townies will have them. Maybe the same could be done with mobile phones? It could make the game more interesting if you decide you want to get your Sim together with a townie that only has a house phone meaning sometimes you might not get through to them at certain times of day.