Normally when I say "I don't have to do that" (meaning I'm not obliged to), I find that as well as putting heavy stress on the word "have", I pronounce if haff.
Is this common? If so, why does the trailing consonant change from "v" to "f"? Are there any other contexts where this or a similar change occurs?
Edit: It's starting to look like have to in the sense of "must" is almost a completely different verb to the standard have. That allows it to conjugate differently, which explains why in "I had to to that", I often say hat.
Edit2: We seem to have identified have, use, and suppose as verbs that can undergo significant changes in pronunciation when coupled with 'to' to indicate a special meaning (the required, habitual, and requested/ordered senses). Per @PPL's comments, there's also the ought/owe pair, and various (often dialectal) changes with some forms of go, want and get. Any more?