Cinebar - the open world and story progression make a difference; it's because of how the townies are actually simulated.
In TS2, townies weren't simulated, only the active lot was. Townies were just data; skill points, relationships, personality points, memories, etc. When a townie came to a lot, community or your Sims' home lot, the correct Sim was spawned to represent the data. When they left, it had been updated with any changes that were made on the lot to things such as their relationships.
What that meant was that when townies weren't on the lot you were on, they didn't even really exist. They weren't home, at work, at a community lot, or anything like that, because only the lot you were on was being simulated. They were pretty much in limbo. So when you called them all the game had to do was check the variables; time, relationship level, etc, and throw either a "no" message or a "yes" message. Townies didn't even need to have phones; notice you could call them at work even if they didn't have a cell phone. I've never tried it, but I would bet that if you removed all the phones from that Sim's home, then called them while they were at home, they would still answer.
In TS3, with the open world and story progression, the townies are being simulated. They can be anywhere; at home, at a community lot, at work, they don't even have to be on a lot, they can literally be anywhere in the world. If you've ever texted a Sim with the smartphones in UL, then found that Sims and watched them, you'll see that they actually get the text tone with the music notes coming from their pocket, and they go through the interaction to check their phone. Sims in TS3 actually have to go through the interactions to answer, so they all needed a phone, so you were able to track them down.
Now that's not to say they can't come up with a new method for TS4 that allows TS2 style phones; they might be able to, and personally I hope they do. That's just an explanation of why they needed phones in TS3 but not TS2.