Even if they did change the frequency the call was on, it would be a simple matter to decode how the frequency change was negotiated, and "follow" the call (also easily accomplished with cellular calls). Failing that, there is a very limited range of frequencies allocated for cordless fones, and simply re-scanning for the conversation is a trivial inconvenience. //cerridwyn// Most of those systems do also change the order of the transmitted data, and that's not limited to a few possibilities. If it's digital, they usually encrypt it (only weak, but hey, you normally have to find the key real time!)
Right. After posting that, I realized I forgot to specify I was only referring to analog cordless/cell fones. Digital is a bit of a different story, as it requires more sophisticated equipment to decode. Still not secure though. (right now it's simply obscure). //cerridwyn//