Yeah. I had this same issue this spring and they came out to clear the line with a water jet. There's a camera on the snake they feed down the line so you can see what's going on. In my case a tree root had grown into the line. (I had old clay lines). I just used roto rooter but it sounds like people here have good luck with united. I assume all of them would have similar services. Surprised the people you called didn't check your outside pipe. The bubbling toilet seems like a dead giveaway of a plugged line.