I've a problem deciding where to place a certain table field within my database.
The database is for a classified ads website.
I want registered and non-registered users to be able to post ads. If an unregistered user posts an ad, the site should ask him for the contact phone, if the user is already registered, then the contact phone stored in the users' table should be used.

Now my question is, would it be possible to store the contact phone for both registered and unregistered users in the same table field?
If so, where should that field be put, in the Classified ads table, or in the users' table (noting that each user within the table has a unique Id, thus, filling the users' table with unregistered users just to get their contact phone will just fill the table with useless data)

Thanks in advance !