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.
If anything that makes it more important. We already know how essentially all the Democrats are going to vote. A couple of Republican defections is all it takes to hold the Senate.
The key here is they don’t have to defect on everything. Turning every Republican senator into a never Trumper isn’t happening. But making some of them defect some of the time is a lot more feasible, and requires constant pressure.