Should I forgive him?

You are too paranoid , of course there is a chance that he is going out with someone else and didn't want to be disturbed. However if you like him you should trust him.

If your number was in his phone and he left it at work, chances were good that he didn't have a way to call; most people don't bother to memorize phone numbers anymore. A hard copy of your phone number would have helped, but even then, he may not have had access to a phone without having to embarrass himself (asking the neighbors or something).

It's only been a month. Not all guys dote on their lovers, and perhaps he's preoccupied with other things. Still, it would have been nice if he'd called you. Tell him that you've noticed that this was the second occasion of a missed plan. He'll be more likely to avoid it in the future if he knows that you're paying attention.

You need to ask yourself some important questions. Just think, if this is how it is now.. won't it get worse? The longer you let things happen, the harder it will be in the end..

I don't think you're being paranoid, I think your gut has already given you the answer. If in fact he has left his phone at the office, which I doubt, he could've found a way to contact you, even if it means driving to you! Him leaving you 4 messages shows that he's feeling guilty... Good luck!