Tupiza is the other way into the salt flat, and the better one: instead of starting at Uyuni you ride south-west across four days of red rock, the Sud Lípez lagoons and the Siloli desert, and reach the salt on the last morning. Tupiza Tours sells that crossing, in both directions, and it is what the town is for.
They also run the thing Tupiza is otherwise known for — horseback riding through the canyons and cactus country around the Quebrada de Palala and the Valle de los Machos, which is genuine Butch-Cassidy landscape — and, improbably, an annual triathlon.
Three offices, in Tupiza, La Paz and Tarija, each with its own mobile; the Tupiza one is the WhatsApp.