The most effective phone calls take less than a minute: say your name, your city or ZIP code, and what you support/oppose, maybe a sentence on why. You’ll be marked down on a spread sheet that is discussed at the daily office strategy meeting.
Other actions like brief emails, meeting in-person at the district office, meeting in-person at the DC office, can also be effective, but take more time and energy. Emails aren’t always read right away like a phone call must be answered for example. And you generally need to make an appointment for an in-person meeting.
Counterpoint: call them anyway. Gum up the works. Change the mind of their interns. Send enough letters that it becomes a waste disposal problem.
Be annoying and difficult and make them work for every inch.
But also be nice. The interns are people too.