I have a blocking device on my phone where if they call more than once, they get the BIG block..If you don't have it here are some options.. I like answering their questions with  questions.
1. after answering, leave your phone line open they can't disconnect: open lines are fun, Put the phone next to a running vacuum cleaner, a loud music speaker using really annoying music, try a garbage disposal, let some kids talk on the phone.
2. Call your law enforcement & report harassment or phone stalking (which are both against the law) you already have their number.

These people call about 4-5 times a day. they do not leave a message and when you try returning their calls they do not answer!!!

They called my minor daughter 50 times in 2 days... what app do you have?

Some one from this number calls several times a day and no one responds when I answer the phone

Many calls from this number, never leaves a voicemail so I figure it's not important!