There is something wrong with all of the writeups in this node. As an ex-
cellular engineer, I feel the need to set some things straight.
artemis entreri: this cannot work. After the voice is done telling you to hang up and the loud beeping stops, power is cut to your landline phone. It will not be reconnected until you hang up or unplug the phone for bit. Technically, there will still be a small voltage applied to your line to detect when you finally hang up; this isn't nearly enough to operate your phone, which wants -48 volts DC.
Even if you hooked up your own -48V power source to your phone, taping the ear and mouthpiece of a landline phone to a cellphone will cause serious harmonic distortion. Even purpose-built acoustic couplers introduce significant distortion at higher frequencies, which is why recent modems connect directly to the phone line.
Any: soldering the ear and mouthpiece to the cellphone electronics won't help either. The impedance will be completely mismatched if the devices aren't outright incompatible. The landline parts most likely work at -48 volts; I'm willing to bet no-one made cellphones that could drive those parts.
BlueJayW: GPS is not necessary to locate a cellphone user, but being able to locate a cellphone user is necessary for the network to work correctly. The network has to know how far you are from the towers nearest you in order to assign your call to the 'best' (usually nearest) tower and tell your phone what power to transmit at. Always using the lowest possible transmit power at the phone and tower allows what's called frequency reuse. Cellular radio would not be possible without frequency reuse, there would not be enough bandwidth for everyone. By transmitting at low power, you limit the reach of your signal and allow someone in a different cell to use the same frequency at the same time. If cells are small, you can get high call density, and at low power you get good battery life. The network tracks your phone by taking repeated measurements of your signal at several towers, and reassigns you to a different cell when you move.
Also, you misunderstand part of the narrative you refer to. A call can always be monitored completely at the telephone switch your tower is connected to. In an analog network in which tower triangulation wasn't possible, whoever was trying to locate a cellular user would need to use a mobile directional receiver to find the user within the cell. They could only hear the tower side of the call until they got close to the phone they're tracking because the tower's signal can be received in the entire cell, but the phone's signal cannot. They would know they were close when they heard the mobile (not tower) side of the call, but the whole call could be recorded at the phone company switch.
In modern US systems, a law called CALEA mandates that the phone company be able to tap any digital or analog cellular call and forward all voice, data and call control information in a standard format to an evidence collection center for all calls to and from any cellular number. All law enforcement needs to do is get a phone number and a court order. If you feel like being paranoid about something, this is a better target :-)
All: The only good way of using modems over cellular is with support from the phone. Even then, 9600 baud is the most you'll realistically get without using technologies such as GPRS or 1x, which use different digital transmission strategies from voice calls and require additional capabilities in your phone's radio and software.