You can try in future to let a male (father, brother, male friend) to take the call if you suspect it's from that pervert. Let them answer the call and ask them to use any vulgarities they can on him

. If not, refer him to police. If he calls everyday around the same time, tell that to the police, and let them hold your SIM card for that period of time while you use a temporary one. If that guy calls again, he will kana the police. Even if private no. not able to track, at least he will be deterred from calling you again. Try contact your operator and ask if they can offer assistance for such cases as well or not.
Heard about 1/3 Singaporeans have Giro contribution to NKF every month. Don't know is it a opt-out or what, but I certainly have no recollection if I have to make the same monthly payment. I will go check and if it is, I will opt out. Not that I don't want to help, but for charity, shouldn't it be a volunteer basis? I am already regularly donating to Red Cross and other charities, I want to be aware and keep track of what charities I am donating to.
If NKF is a caring charity, given its success, it should also be more magnanimous in helping out other charities. Like giving guidance on how to organise effective fund-raising events, etc.