There is no specific tower that it would use, it would just use the closest one to where ever you are at, at that particular time. As in near home, school, work, store, etc., and the 'towers' would be ones either belonging to or used by your service provider/carrier. So which ever service provider/carrier you have, it is theirs that get used.
If you are getting bad service, you should first make sure your firmware is fully updated on the phone, then update the phones PRL and/or Profile (should be in the same area as the update for firmware) to make sure you are connected to the tower nearest to you.
If none of that helps, you need to check with your service provider/carrier.