I will never give my phone no to BG for safety reasons.  For really nice BG that I want to re-visit, I may ask for their phone no and call them if needed.  You can use different SIM or nickname or for me I use secured database in my PDA/computers to store them.

agreed and it is always a danger to give phone no to girls other than your wife or girl-friend as it is hard to control when they will call you.  Also, there is no point to keep their phone no. either.  So usually I will throw away immediately girls phone no, mami's business card when I leave the sauna and nightclub/KTV.   Even for some really good friends or mami, I will keep their phone no in my computer with password protection and only used my secret second phone to call them with hidden caller ID.