As a general rule of thumb, you should strive for the most personable method reasonable and appropriate. Business is built on relationships, and the more "face time" you can get with your contacts, the stronger those relationships will be.
anup gives some good guidelines on what's reasonable and appropriate, but I think it's important to note that this is not a place to follow the golden rule. What's right for you is not right for everyone else.
For example, just because your day is booked to the minute doesn't mean that your client is too busy for visitors; sometimes having some comapny can be fun. On the same token, if you have a client who never leaves the office, he may not even want to hear your voice on the phone.
Your best bet is to feel them out: how much resistance is there to an in person? What about a phone call? You also need to balance this against your time as, generally, your contact will send signals if you're spending too much "bs" time with them.