How about "Speak intelligibly, or GET OFF MY PHONE!" (notice I don't suggest, "Speak intelligently . . .").
Note: I realize I risk being branded a xenophobe here, and I don't deserve the label. I simply know that when I've been in a foreign country, if I couldn't communicate clearly in the native language, I wouldn't inflict my ineptitude on others--at least, not unless I absolutely had to.