No, it isn't. And BTS segmentation is mostly done for reducing the number of phones to service, as each segment is handled as a separate cell. But the whole point is usually moot, see next
It is not even available to then usually, as it depends on Timing Advance which is handled inside the mobile equipment as well as the BTS, but only for the servicing BTS. And there's no means in GSM to remotely force a single phone to do an handover to another BTS. So without the phone actively participating, you usually have the TA for one BTS only, and providers don't even have access to that parameter of a remote BTS. To do a triangulation based upon timing with more than 1 BTS and without support from the ME, you need the non-servicing towers to act as monitors listening to the ME signal sent to the servicing BTS, and comparing this to a global synchronized time raster. This technique isn't implemented in BTS usually. Ergo no triangulation, except when executive forces come with special equipment to do this. (a special case is when the phone just this moment decides to do a handover, then you actually get TA for two BTS (but again afaik the BTS can't be queried about TA for any ME it services), and thus a really good idea where the phone is this moment. All above is for 2G/GSM, for 3G/UMTS WCDMA things might be quite different).
For doing triangulation with ME support see http://lists.openmoko.org/pipermail/...ne/002987.html