It is pronounced /ˈhæf tə/ when it acts as a semi-modal, equivalent to must, AND the two words fall together. In all other circumstances it is pronounced as /ˈhæv tə/.
ADD: Actually, the to in /ˈhæv tə/ will in most cases be pronounced /tᵿ/, but that's a very minor point.
I /ˈhæf tə/ tell you the truth: I have no idea what you're talking about.
I /ˈhæv/, /tə/ tell you the truth, no idea what you're talking about.
We /ˈhæv/ /tə/ the left, the Colosseum; /tə/ the right, the Arch of Constantine.
We /ˈhæv/ /tə/ that end instituted a new policy.
I /ˈhæf tə/ go to Ottawa tomorrow.
?I /ˈhæv/, /tə/ my dismay, to go to Ottawa tomorrow.
You're very unlikely to hear the last one. It's not formally "incorrect", but only a very literate speaker or writer, familiar with similar uses of full modals, would think that have to might be be deployed with an adverbial between the auxiliary and the lexical verb; and only a speaker or writer with a tin ear would allow himself to do so.