Short story about a man getting the wrong phone number (with old-style exchange) and being 'disappeared'

Question

An old science-fiction-y story, at least 1970s, probably earlier. Silly, short, kind of like one of Ray Bradbury's throwaways or one of Phil Dick's goofier renditions.

I think but am not sure that the name of the story was something like EXchange-2-3566, where exchange is of course the old-style word/location telephone exchange. The exchange, I keep thinking, was 'boojum', 'snark', or 'jabberwock', or at any rate some other Lewis-Carroll-associated beastie, but I am not sure because if that were the case it seems like google should be able to tell me and it doesn't seem to know.

The story was about someone trying to contact a particular exchange with the same first two letters as the one in the title, getting the wrong one (presumably due to 'operator' — in this case literal operator! — error), and being misidentified as part of a secret group. And by the time that they catch on that the bewildered person isn't part of the group, it's too late, and they have to make him disappear. Possibly 'softly and silently', as in Boojum, and we have come full circle.

Answer 1

Mugwump 4 by Robert Silverberg.

The story starts:

Al Miller was only trying to phone the Friendly Finance Corpo­ration to ask about an extension on his loan. It was a Murray Hill number, and he had dialed as far as MU-4 when the receiver clicked queerly and a voice said, "Come in, Operator Nine. Oper­ator Nine, do you read me?"
Al frowned. "I didn’t want the operator. There must be some­thing wrong with my phone if—"
"Just a minute. Who are you?"
"I ought to ask you that," Al said. "What are you doing on the other end of my phone, anyway? I hadn’t even finished dialing. I got as far as MU-4 and—"
"Well? You dialed MUgwump 4 and you got us. What more do you want?" A suspicious pause. "Say, you aren’t Operator Nine!"