So we have a cell Central Office layer that is regional and connectivity to it would be necessary for individual towers to complete calls. Let me extend the Q to ask: is there some standard practice that confines geographic placement of COs to a certain radius? How many of these (as opposed to mere towers) would we find on a map, if such a map was available? I presume that if a CO was isolated no one could roam-in because the necessary central inter-carrier auth could not be completed, but what of existing subscribers? Would a CO facility, even if it was restarted from power down, retain enough subscriber data to bring its 'native' users in the local area to the point where that can complete calls to each other?
Sorry about the Wheeler (FCC Chairman) booboo in the summary. Brain fart.
If you want a map of all the COs -- they are here : http://www.dslreports.com/coinfo They are not placed by geographic radius, but by number of subscribers. Back in the day, a central office might serve an exchange or two (an exchange is the three digits after the area code in a phone number, for example 517-355, where 355 was the exchange). Of course some COs were larger and served multiple exchanges, some getting as large as a dozen and some were smaller and only handled a single exchange. Each exchange could have just short of 10,000 subscribers (known as nodes, corresponding to the 4 digits after the exchange in the phone number).
COs, regardless of the brand (two of the most common in modern day were the 5ESS and the DMS100) knew ALL the info for their subscribers and how to route calls to tandem (directly connected) switches and upper class switches. These were known as Class 5 switches (they had directly connected subscribers) Similar to IP routing, if the phone number you were dialing was not a local subscriber then it would switch the call to the next higher class switch (Class 4), who knew how to route calls to every exchange in your LATA (your toll-free calling area). If it didn't know how to route it, it would toss it to the Class 3 switch and so forth. Billing is always done at your local CO using "CDR" records (and sent to your phone company for central billing). There are now exceptions to these roles with LNP (local number portability), but the same series of events generally occur. Remote COs know nothing of subscribers in other COs.
So, short answer, if a CO powered down completely, calls within that exchange would not get delivered. If your CO survived but was disconnected from the CLASS 4 switch, then it would be able to process calls locally and be able to send calls to the tandem switches, but you wouldn't be able to call others in your LATA and they wouldn't be able to call you.
Now cellular is a totally different game altogether. Cellular companies are subscribers of the phone network, not really a part of it. They run their own infrastructure and don't directly participate in SS7 for routing. A CO could disappear and the cellular network wouldn't necessarily be hurt (unless that was their point of termination with the phone network).
These were a lot more words than most people will care to read for a comment... I spent 8 years on the 5ESS DSIG crew installing new COs and working on the SS7 protocol.