This topic comes up regularly. Bound to be some searchable old threads here about using a baby monitor for a wireless link, but offhand I don't recall anyone successfully building an ecaller around one. Two reasons why, other than the legality question immediately come to mind:
a.) the audio frequency response of a baby monitor is not likely to be anywhere near as good as what a good wireless microphone tx/rx set offers. (the human voice is easy to reproduce, high-pitched rabbit cries and mouse squeaks are tougher, requiring much higher fidelity equipment).
b). the signal to noise ratio is unlikely to be engineered for further amplification, leading to noticeable background static.
Then too, there's the ethical question of would you have wanted someone to have stepped on your baby monitor frequency yourself--with the god-awful sounds of a rabbit getting shredded--when you were using it to monitor your infant? Even if this sort of thing happened only once in a blue moon, any such occurrence wouldn't reflect well on our sport.
Wireless microphone frequencies are over on another frequency band,(~170MHz to 215MHz) on a VHF sideband which they've mostly had to themselves until recently. Until something better comes along, they're still anexcellent choice,provided you do a little monitoring before choosing a frequency. (Digital TV is nibbling at them, there are a number of threads in the archives and a tutorial off the tech page discussing this).
LionHo
