What is he thinking?

I have to say congrats on taking the first step like WindChaser said. I wish more women would do this since if would make it a lot easier on us guys. =P I would really try to call him. I know if you feel like there is some chemistry there you need to pursue it whatever the cost. Please don't regret over something like this, call him! He might be confused, I know I am like this and you may have to drop him some more hints. I practically have to be hit over the head before I realize what is happening. Hope everything works out for you!

Perhaps. But why did he left a message on your phone and not talk to you directly? Was it because you weren't able to pickup the call? If that wasn't the case, it sounds like he's not sure what to do. He may want you to call him again. And I have to praise you for writing him that note, giving him your number, and taking the vital initiative or green light steps. A LOT of younger girls lack this, which is why I have a desire for older women. Anyhow, you should call him

Now if the case is where he called you and you didn't pick up, then yea! That's a really good sign that a guy calls back WITH his number for you. Both cases seem to be in your favor. Just take this opportunity and go for it. I thank you for doing what a woman should do. Best of luck!

Yes, he is definitely interested. People sometimes leave their phone #s on going-away cards or sometimes when you graduate you give someone a picture with contact # on the back. He could have looked at your # and then decided not to call you. In this case, he wants to talk you outside of work. He took the time and the initiative to go ahead and contact you. He wouldn't do that if all he thought you were was a friend from work. Call him back and ask him how things are, if he wants to catch a coffee after work etc. Good for in taking the initiative. And it's nice to hear that he called you back!