When leads are imported into your account and called and/or texted prior to having a phone number in their state, they are then automatically assigned the default phone number.
To un-assign leads now to that number, navigate to the Phone Setup page in the left hand menu, and scroll down to view your current phone numbers. You will first want to at least temporarily set a different phone number as your default phone number using the MORE button beside another number. You will then find the MORE button beside the previous default number to choose. Select "detach from leads," and then you can set it once more as your default.
Now the next time you call or text those leads, they will be reassigned the phone number in their state for future communication.