In the US, the federal government requires cell phone companies to know where cell phone users are. This is so that if a cell phone user calls 911, the police/EMS/etc can find them.

Some cell companies perform this look up via cell tower triangulation whereas others do it via GPS. I want to know what companies/phones do this with GPS! And I’m curious what % of cell phones in general have GPS built in!

Let’s say I have a Verizon cell phone, my friend has a Sprint cell phone, and my other friend has a Cingular cell phone. We all have the same area code. Is there a way to know if the three phones will tell the same time without actually putting them side-by-side and comparing them? Do cell phones use a cell tower to get their time? What do cell towers use? Do the various major carriers all use the same source to set their clocks?
Thank you for the answer re: networks recording when calls are made. Do all networks (e.g., Verizon and Cingular) synchronize off the same time source? Is this something regulated by the Government?