You'd need to be lucky to catch much with only 40 mins of broadcasting from each charge, but at that price if you have any sort of plan it's worth a punt surely.
Weren't you the same guy who was trying to stream audio over ethernet or some similar setup a while back? How'd that work out for you in the end?
i'd like to know if there is any security feature like an access code which has to be typed in before one can actually listen to the sounds. otherwise someone could "accidently" call that box and listen...
and as always, the problem is the too big and weak battery
A few things that you need to be aware of with these types of devices:
1. The use of GSM will be horrendously expensive unless you have a very good plan and you can make calls for next to nothing.
2. Be aware that even though you may very well be in the next room, the device has to be able to rx/tx to the cellphone system it is registered to. A quick test call on you mobile at the location and level from the floor that the device is expected to operate from. If the soft toy is going to be left on the floor next to the bed, then a test call from there will confirm if the system will work.
3. GSM is notorious for causing interference to other electronic devices in the vicinity, ie tv's, radios, hifi's, ipods, other phones etc.
4. Unless the device is going to be used for a "once only" event, you have to plan to extricate the device, charge it back up, then re-insert it, all without alerting the suspicion of the intended victim.
5. Battery in these devices will always fail when you have to depend on them most.
6. Mains power is more reliable, but trickier to plan around. A well hidden device in an everyday object can be left in place for a considerable time, but remember point 3 re interference.
Hope this helps out anyone else who might be thinking about this type of device.