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.
Thought I’d add that mass phone calls are a tactic used by the NRA and other powerful lobby groups. Getting a lot of their group’s members to voice their opinion is the key to their success…perhaps to the point of making an office fearful of the backlash via phone calls for going against them.
Relevant John Oliver segment
The real power of the NRA is their membership is mostly reliable voters.
It also helps if you have a very specific and narrow cause … muh guns rights … and anti or pro specific legislation. This creates broad consensus… that follows the NRA draft talking points they usually bring out.