Meridian Mail is a voice mail system by Nortel which was installed in the WPI residence halls during the campus networking project in the summer of 1994.
The phrase "Meridian Mail. Mailbox?" slowly became engrained in students' heads thereafter.
Voicemail at WPI works via the standard Meridian Mail system. To access voicemail, users dial x5900. They are prompted for their mailbox number and password, each followed by the pound sign. To call directly to a voice mailbox (normal calling or transferring to voicemail), express messaging is used via x5901.
The woman who guides you through the prompts is Joan Kenley.
Joan Kenley does voiceovers for several phone systems (including work for Verizon and Sprint). Among college students, she is perhaps most recognized as the woman saying "The fingers you have used to dial are too fat. To obtain a special dialing wand, please mash the keypad with your palm now" from The Simpsons.