Dial your number from a landline, press # then your pin.
Quick response and to the point, thanks tjk629!
I'll elaborate for future reference, if that's ok (this will perhaps be "Well, DUH!" for most readers, but better to have a External Voicemail Access for Dummies guide than suffer extra questions):
Call your mobile number from another phone (yes, landline works best), but when you get your outgoing voicemail greeting on the line, then press # followed by your 4-digit voicemail PIN. Like tjk629 said...
Also keep in mind that you're calling yourself, so if you've roamed outside your mobile phone's normal home area code, then you're dialing yourself long-distance, technically.
You'll then be in the audio voicemail menu with instructions to follow, which is a little jolt if you're used to the VZW Visual Voicemail feature. If you're like me and have a bunch of old messages hanging out in the mailbox, you'll be prompted to save the ancient messages or allow them to be deleted, and you may have to wait through multiple recordings until you locate the new message you want to hear. Just be patient with the automated system and you'll get to that important voicemail.
An extra hint (helpful maybe)... I found I didn't like the sound of my outgoing message (which is obnoxiously long since it's the front door to my mobile tech support business) when I recorded it directly from my mobile phone. So I actually used this external access for voicemail from my landline to get the "record a new greeting" feature from a good-sounding wired handset. Just another reason I haven't dropped the home landline... yet.