I’ve always wondered where the devoicing of the v in ‘have to’ came from. I assumed it was assimilation to the unvoiced following t. Just now it dawned on me that when you insert a word between have and to, the v remains voiced, as in, “…you have first to get….” That tells me it is the assimilation to the t-. With a similar case, used to, it is hard to think of a word that can be inserted between use and to.