On first contact to check who they are (phoning or first entering the building) I think it's generally best to address them by their full name (e.g. "Is that Alison Smith?", "Can I speak to Alison Smith?" or "Hello, Alison Smith?") After that if I'm honest I usually manage to get away with not calling them anything, it's not generally necessary to keep repeating someone's name during a conversation (it's actually rather an irritating modern habit that bank managers and telesales people seem very fond of - there's nothing rude or wrong about saying "you." provided the rest of your speech is polite.), eventually you'll pick up a clue about what they prefer to be called. If you absolutely have to call them something go with Mr for a man and Mrs for a woman - if you're wrong about their title they will correct you, and most women would far rather be mistaken for a Mrs than a Miss. (some women also dislike Ms. and it does sound a bit ugly said out loud), or they might say "Please call me Alison." Never, ever, ever say "May I call you (first name)" - if the person doesn't like it it's very awkward for them to answer "No you may not." and doesn't create a good impression if they're the more formal type. These days most people are happy to be addressed by first name only, but as those that aren't tend to be those who set more store by propriety, it's best to play safe.

I agree with everything Lady Fuschia said, other than addressing a woman as Mrs. I would suggest Ms. You can't tell if someone is married and Miss is too young sounding for a professional woman. I work in a very large law firm, and all of our name plates say Mr. So-and-so and Ms. So-and-so. I think Ms. has evolved from the "feminist" connotation that is started with into the standard professional title so as not to have to figure out if someone is Mrs. or Miss.

Good question! Go to the company web site and look under the "about us" tab. Or you could call and ask the receptionist how they spell their name. As for the Mrs. or Ms, I just us Ms. like I would use Mr. I think most women would understand this.