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You'd "get a roundtrip", i.e. hear the response from the mobile only if the phone transmits on a frequency you'd be listening to (Your local cell tower) AND the signal from the mobile isn't attenuated or covered under radio interference from other phones.
In that case, if you've managed to identify a particular radio signal as your target, then you could simply employ old school radio cross direction finding.

Oh, all this is with regards to TDMA networks (like GSM, for example). 3G uses CDMA, where one needs to know the individual key, in order to separate a meaningful signal from the surrounding noise.