Given a state in which the cell towers are down and out of function, there is no internet connection, would WebRTC be able to form peer to peer connections?
Such that in case connection is lost to a server due to loss of internet connection, there is a possibility to communicate between all devices with software containing peer to peer connections made through WebRTC without being connected to cell towers or the internet.
There could be a couple of different ideas mixed into your question, so it may be worth being quite precise in the answer.
WebRTC is IP and HTTP based, as mentioned in the comments, so it does need IP connectivity, but that connectivity could be a private network and not necessarily the internet, if both devices were connected to the same private network, or to two interconnected private networks.
However, you also say that cell towers are down - in this case there would be no connectivity to the cellular network, so the devices would only be able to communicate if they both had WiFi capabilities and were connected to the same WiFi networks, or to separate WiFi networks that had some sort of connection between them (I'm ignoring wired connections to the devices as I doubt this is what you are thinking of).
If you meant, could phones connect in a peer to peer manner if the network went down so they could work together to make and route calls, this is something which has been discussed and looked at quite extensively and certainly chat apps existed and exist which can do this, possibly the best known being the now discontinued FireChat (https://en.wikipedia.org/wiki/FireChat).
Its a constantly evolving field so any links will age quickly, but if you search for 'Mobile Mesh Networking' you will usually find the right material.
And, in answer to your original question - WebRTC itself does not enable peer to peer connectivity.
