Unused area codes I can think of now: 708,719,908,909,917.
(706 can be used for some Mexican points?)
This excludes N10 and N11 codes. Also, I have seen 260 mentioned
in "blue book" (via this Digest, as I am not sure what "blue book"
is, let alone seen it) as NNX area code when that becomes necessary.
Presently, 1260xxxx within Maryland (and beyond Severna Park calling
area) is a call to the 260 exchange there (could be Annapolis at
Severna Park rate), so area 260 would require changes in dialing if
the system is to remain free of timeout for direct-dial calls.