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.
The call is from Morning Star, a collection agency. I suspect you have some upaind bills that have gone to collections. "XO" is just a telecommunication company (like AT&T), not the name of the party who is calling.
Morningstar is not a collection agency. It's like an online Wall Street Journal. They want to sell subscriptions to their online mag. Morning Star is a reputable and highly thought of investment tool, but why they're doing this junk makes me wonder. Also makes me not want to by. Jeez.
I received the same missed call on my cell as I use that for anything I sign up for online. I did sign up for a yearly subscription for morningstar.com to track and manage investments. It was about a year ago so it must be renewal time