Why isn't my Google Voice account linked to my Gmail account?

Only the Google Voice account that shares your Gmail username will be linked for making or receiving phone calls in Gmail. For example, if your Gmail address is example@gmail.com and this is the same username you utilize to sign into Google Voice, then you'll already be linked.

However, if your Gmail address is example@gmail.com and your Google Voice account username is example@domain.com, than you will be unable to link these accounts. At this time, it's not possible to transfer Google Voice accounts between Google Accounts.