True. But the connection point into Level 3 or Sonic is known, and could be passed on.
If the call is injected at one of those points, then yes, Sonic or L3 could determine the originating IP address. If the call is injected into the PSTN upstream, then neither would know the origination.
Even if Sonic did have the origination IP, that doesn't guarantee that an incoming call with an area code that's not geographically consistent is necessarily spoofed. For example, a Sonic member using the Sonic Mobile Communicator app could be calling legitimately from the East Coast, using a number based in CA.
In addition, even if Sonic could determine the caller's true location, the protocol used to send the caller's information to the called phone doesn't support anything other than sending the caller's number and name.