Mmm.. that's pretty unlikely in reality, especially given the 200 times/sec channel hopping that (most) GSM systems utilize, on top of the PSK modulation and code spreading.
More likely is some internal thing in the specific handsets that happens to resonate just right, like how I can "hear" the Core2Duo in my notebook under certain loads.
If that is the case then why does every single GSM phone that I have ever come across make exactly the same chirping sounds when placed next to anything with an unshielded amplifier when it checks in or receives a text message ?