You are wrong. "Their" was the proper usage. It indicates the possessive. "They're" is a contraction for "they are" which is not what is being stated in Mark's sentence.
I got the same call everyone else got at 9:48AM this morning. I'd want to shoot the caller like Woodie Harrelson does it in the movie Zombielnd. Double Tap right between the eye balls!
It is Morningstar - just call them back at the number and ask them to remove you from their calling list. It is a painless and efficient manner to deal with the problem rather than whining about it here (though I am grateful that you have - because it helped me identify the number quicker)