These devices are very illegal. They are also very poorly built and freqently sources of bizarre interference to 800 MHz trunked radio systems as well as to various military UHF comm systems.
When I find them, I smash them in front of the owner, and discourage him from installing any more of them. It helps to have an MP along.
In addition to being illegal interference sources and occasionally locking up cells, they also aren't very effective. You'd do much better with an analogue bag phone with a yagi on it (which is pretty much the standard home telephone in some places around here). The analogue systems have considerably better intelligibility as well.
"C'est un Nagra. C'est suisse, et tres, tres precis."[TELECOM Digest Editor's Note: I do not see what would be so illegal about hooking up an external antenna at a somewhat higher line of sight. Many phones (Nokia models for example) have a plug on the back side for just that reason. PAT]