From the book "Gem"
1-You ....phone him before 3 pm .He won't be available after that
(Must-have to-don't have to-hadn't to)
2-You ....phone him now.It's very urgent
(Must-have to-dont have to-hadn't to)
I think the first is "have to" because there is an outer conditions
The second I think it's "must" as it's an individual opinion so it's an advice