I actually built this product out (for both Uber and Lyft) - SMS/MMS-based ride requests (MMS only for sending google maps PNGs to confirm pickup location). I was about to convert it from a personal project (I used it to get rides on both services for a few years) to a "real" one open to customers, etc and then ran into snags with both APIs at around the same time. Uber's API changed to potentially require a web flow to confirm surge pricing (IIRC - now it's scheduled to be disabled entirely very soon) and Lyft's API was basically shut off entirely (there's a way to apply but I was unable to make any progress even through internal contacts at Lyft). I'll try and keep an eye out for any changes...