The technology to route calls inbound to an 800 number to different destinations based on location of caller, has existed for going on two decades now. They should probably apply it to the Clearance Delivery number with a "bail out" method for the inevitable few mistakes it'll make.
Calling from an area where you need to talk to the TRACON? Call triggers an announcement that it appears you need to talk to the TRACON, direct number is blah blah for your reference, remain on the line for the TRACON, or press zero if you still need to speak to National.
This crap isn't hard nor expensive at all. Wasn't even hard back in the early 90s. Does require some brainpower to design at the carrier(s) involved and can even be re-routed if a location goes unavailable at certain hours of the day, or whatever.
Jumping to WXBRIEF for a minute... I find their use of voice recognition to be wholly annoying. Airports, terminals, and ramps are noisy places, and I've been routed to crap I didn't want just by background noise. I'd really rather they dump that and just do DTMF signaling. Calling FSS doesn't have any real requirement that it be a "hands free" operation for anyone other than a moron calling them while driving, and you're not paying attention to the briefing properly if you're doing that anyway.