I suspect that they aren't wildly concerned:
For users on contract or fixed-price month-to-month, carriers often have an incentive to encourage them to use wifi(unless they think you'll upgrade to a more expensive data plan, or get whacked with overage fees, the less data you use the less you cost; but you still pay the same for the service). So long as they want to continue paying, the carriers would probably be delighted to have them drop off the grid and go mesh out to their heart's content.
Also, internet access in itself doesn't provide a phone number(though you can generally get a VOIP line more cheaply than a cell or landline), so only users who don't actually phone with their phones, or are willing to have phone access only when within range of the wifi or mesh, or who are willing to put up with having both a cell and a VOIP number, are likely to jump from their voice plan.
Plus, wireless meshes can, unless conditions are good, exhibit some pretty tepid latency and packet loss numbers. Well worth what you (don't) pay for bulk data transport; but cuts the utility for latency-sensitive applications.
This is hardly to say that meshes are useless(indeed, they are pretty neat, and certainly a good thing to have in place for resilience purposes and various other things); but they aren't a terribly effective direct competitor to contemporary cellular data standards, or to a one-hop wireless link to a hardline of some sort.