As I understand "how GTD is supposed to work," you have a few options.
This can be a project or a single item.
NA: @Home: Find out what time dental office opens on Monday [via internet, Rolodex, phone book, or calling (so then it's @Call)].
Plans: After determining when office opens, add to calendar an item to call office 10 minutes after it opens on Monday (if it's open on Monday).
Single Item: (assuming that you already know what time to call on Monday).
Add to calendar an item to call office 10 minutes after it opens on Monday.
If you won't be able to determine over the weekend the time to call on Monday, you instead have an NA that is @Call: get dental appointment (not doable until Monday).
How's that sound?