Cold calling proprietary search tips?

I've been running some proprietary efforts over the past 3 months, mostly physical mail which is having fairly low response rates so far. I want to incorporate calls for these targets after I send out a few round of letters, but I don't have any experience here and would love to learn more about what's worked for others. Does anyone have any good tips or resources for cold (or warmish) calling? Anything is greatly appreciated!