HOW DO I GENTLY RELAY THE MESSAGE I'M NOT ON CALL 24/7?

Hello Active Rain Community! I have some nice customers that were happy with my work back in Sept '08 and have since referred me several others. It is customary for them to sometimes call me evening hours for a quick question or big problem (real estate related of course). In the past, I have answered the phone at inconvenient times because I knew the possible importance of the issue, yet it could have waited till the following day. However, last night "pushed the button". They called at 6:30 p.m. (leaving a voice mail to call me as soon as you get this message) but I couldn't answer-dinnertime with the family, and then again called at 8:30 saying you can call us anytime up until 11pm-"we're up". Well, I would be up too but I feel there has to be a line where real estate ends and family/personal  life begins. Any suggestions how to get this point across "gently" without hurting feelings and/or losing good customer relations?  Also, what's your opinion?