This is not as dumb as it may seem, but in fact two separate operations.
When you press Feature 986, the Voicemail prompts you for a mailbox number, not an extension number (or DN)
Your one-touch speed-dials are programmed as on-touch extension dialing (DN)
Get it?
You phone technician assigned the voice mailboxes to be numbered the same as the extensions for simplicity, but make no mistake, extension 223 is much different than mailbox 223.
I can set my extension to go to voicemail after so many rings, but the benefit of feature 986 is to put the call directly into a mailbox, so it asks you specifically for a mailbox number.