Anywhere that GPS is used in a timing-critical application, they'll be using a timing receiver (like http://www.trimble.com/timing/thunderbolt-e.aspx) to handle their time reference; those use the GPS signal (when they have it) to discipline a (ovenized) quartz crystal, so that they have good performance during "holdover" (loss of GPS) especially for short-term outages. In its default configuration, the Thunderbolt-E linked above, for example, will be within +- 8us after being in holdover for 24 hours.
There is certainly hardware available that can do better than this, but the parent asked about cell sites in specific (and cell sites are one of the top users of the Thunderbolt). You can get GPS hardware based on atomic references, for example, that will do better over these kinds of timespans.
(You really need GPS or some other precision time transfer mechanism for long-term accuracy, though ... there's no getting past that.)
Well...if you're in that position, you probably have money to spend on failover config. Pretty accurate hardware clock modules were available even before GPS. Not cheap, not simple, but reliable - if you have the need and the money.
