The REAL police-based charities don't call for donations.  Fake charities with "police" in their name do call.  They typically keep 90-97% of your donation to cover what they call "fund raising costs" but is actually high wages for the scammers running the operation.

Never donate to a charity that calls you on the phone.  Even the best of them keep 60% or more to cover rental of the telephone boiler room and low wages for the people making the calls.