Here's a cool (although arcane) tool found by a reader:
He writes: "When you put in your phone number, you get a lot of data, but the most fun is the part where it says 'Central Office.' In some areas, such as mine, it still lists the old alphanumeric code for the prefix. For example, my phone number is 503-771-xxxx (in Portland, Oregon). The Central Office field lists it as 'PORTLAND-PROSPECT.' The old alphanumeric code would be PRospect 1-xxxx.
This website did help me find out which phone company had posession of the vanity number I wanted for my business, so it's helpful for that, too."