Verizon seems to be a lot more stingy with letting customers check their voicemail without incurring airtime charges. The landline access numbers are apparently often called "backdoor" numbers. There are many people out there trying to maintain lists of backdoor numbers, but for Verizon they are mostly disconnected.
The best list was found here: He apparently actually tries the numbers and edited out the chaff from the list that is at the other sites.
http://www.bridog.net/cellular/voicemail.txt
The access number for the DC area, BTW, is 301 802 6245
