Telemarketing is expensive. Typically a legitimate charity calling for donations, the company making the calls keeps about 70% of every donation. For charities with "police", "sherrif" or "fire department" in the name the number is much worse. Often the caller keeps more than 90% of your donation.
If you want to donate to the police, call the non-emergency number (its in the phone book) and ask where you can send a check.