Explain the telephones

OK, boys'n'girls. Someone please explain why old-fashioned wired land-line telephones were the gateway instruments for travelling from the matrix to the real world.

At least one character in the matrix had a cell phone. Why couldn't all our heros carry cell phones for easy exits?

Hello people,

Maybe because Matrix system had something to block the
travelling by mobile... and as we could see, they could travel with just a few phones.
The operator had the ability to get through Matrix system and
use some phones to make people travel... but he could not get
any phones that he wanted... Matrix was not perfect, and maybe
he could check the errors in the system to get in it .
With their mobile they could only get instructions from the operator... well, that´s all that I figured out.

I think it has something to do with the connections of the Matrix. Like if the machines had hidden a connection system within the "telephones" system for the agents to use, and the humans used this system to get out of the Matrix. As for not using cell phones, well, maybe they're just not trustworthy enough or are excluded of this system.