Think of the LITERAL interpretation of an I/O signal in Tron (the "I/O Tower") -- that's what a landline is. It's a literal connection to the I/O infrastructure of the Matrix.
I think it's the latter -- I think all landlines are an exit, until it's cut. But I'm not 100% sure about that one.
I'm about ready to say it's the best movie I've ever seen.
I don't think that the "real world" is yet another matrix or other program. I think it really is the real world. If it were just another program, why go through all the trouble that the machines are going through...why not just write a whole other program to replace it with...if it weren't real, why would they need to go through the cycle of waiting for the anomoly of The One to occur so that they can restart/repeat