Keep a record of all their calls - it is actually illegal for them to call and harass - under the Fair Debt Collections Practices Act (FDCPA). 20 calls a day would likely be considered harassment especially if you've told them to stop. Also they can't keep calling after you've asked them not to.
§ 806. Harassment or abuse
A debt collector may not engage in any conduct the natural
consequence of which is to harass, oppress, or abuse any person in connection with the collection of a debt. Without limiting the general application of the foregoing, the following conduct is a violation of this section:
(5)Causing a telephone to ring or engaging any person in telephone conversation repeatedly or continuously with intent to annoy, abuse, or harass any person at the called number.
Here is the rest of the statute .