The best thing to do with a BBS is have a second phone line dedicated to the BBS. That lets you keep the BBS up 24 hours a day, avoids the problem of people calling the number at the wrong time, and is generally simpler. And a second phone isn't really all that expensive.
You can, however, use your main phone line. Let people know that the BBS is available only during certain hours. Bring up the BBS only during those hours, and expect a few random modem calls during the off hours.