If you're not running a special purpose BBS, you may want as many users as possible -- it certainly keeps things interesting. I have two suggestions for getting more callers:
First, let people know the BBS is around. You can do this by calling other BBSs in the area and leaving a message that includes your BBS's number and what it specializes in (if anything).
Second, people tend to like BBSs with lots of interesting files to download. They may then stick around to chat, but the thing that hooks them first is the availability of free or shareware software.