I want to produce a list over the GSM phones in an area. I don't care about what data goes back and forth, neither are the various conversations relevant.

Most phones have IMEI (International Mobile Equipment Identity), this could serve as an identifier. 

Given the fact that phones will operate on different frequencies, is it even feasible with a relatively small unit to sniff the GSM meta data in an area of approximately 30 meters?

I would like to perform this within EU legislation but without the explicit support from any network operator.