How to deal with repeat callers?

Hello. Long time lurker seeking advice (mixed in with a rant).

I have been in my role for over a year now and I am looking for advice on spam /harassment callers. I work in a public facing agency and the past few months I have been struggling really hard with repeat callers. They weren’t as bad when I started but now they are really aggressive and mean.

For example, this past Friday, a gentleman called about 20x. I did not pick up 20x but only 4 times and he said, very aggressively, he would continue calling until someone picked up their phone. He has been calling almost daily for 2 weeks. I transfer him to the department phone, specific people he’s working with, and their supervisor if need be. Another lady, just today, said she would continue calling until someone picked up the phone, but she’s only called twice today but said she has been calling for a week.

In my role, I answer the main phone but transfer people to the department phones or the specified person they are trying to reach. I don’t take messages and they are unable to leave voicemails on the main phone. My supervisor said I don’t have to answer calls if they are harassing or call multiple times.

We are a busy agency and I try not to answer phone calls that are repeat callers who have already been transferred to the correct departments they are trying to reach. Sometimes I accidentally answer more than 3-4x because of how busy the phones get. I write down the phone numbers that call and who I have transferred them to, but it appears we don’t have much of a protocol for callers besides my supervisor saying if they keep calling to not answer them.

Do you have any advice on how to deal with repeat or spam callers besides not picking up the phone?