That depends on what network you're using. If you're using a landline from the incumbent local exchange carrier, the probability of a local call going over voip is effectively almost zero. Likewise, for long distance carriers, AT&T, the ex-MCI networks Verizon owns, and the Sprint wireline long distance network (their mobile stuff goes over separate facilities; their long distance network, as well as most of the ex-MCI networks use a platform called the DMS-250, which is very much oriented to non-packet connectivity) generally don't use voip trunks for national traffic. Also, there is no least cost routing operation like on most of the smaller carriers, so there's no need to hit any sort of public network until it reaches the access tandem at your destination, at which point, it definitely isn't exchanged in any sort of IP format.
Internationally, it depends on what carrier they interconnect with and what they want. Generally speaking, I think Verizon will use more IP-based routes (usually to more expensive countries) then the other two.
By contrast, landline service coming from the cable company generally does go over voip, but only within their internal network. For local and inbound calls, it'll still hit some DSx trunks back to the phone network. 1+ long distance traffic, at least on Comcast, is definitely in IP format, and could very well even be hitting the public internet for least cost routing operations.