The short answer is yes.
Ideally, I like the first few sessions with a student to be in-person. It helps me build a better rapport, which I think is very important. Later on, online tutoring can be equally as effective as in-person tutoring. I currently have reason to believe that online sessions are potentially much more effective, productive, useful than in-person sessions.
As we approach the test date I like to schedule proctored sessions where I observe a student as they take a practice test as if I were an actual proctor. I have not conducted a proctored session online yet, but I imagine it would be just as effective, and possibly even more so.
If you want to have productive online sessions, you need to have the right equipment. I currently recommend an iPad with an Apple Pencil, decent WiFi, headphones, the FaceTime and Explain Everything apps, and an environment with minimal distractions.
You can learn more about my recommendations from my post on the best setup for online tutoring.
After a certain number of tutoring sessions, most of the tutoring starts to resemble conversations. I stop showing students how to solve problems as often as I do at the beginning of the test prep process. Instead, I give them pointers and ask them leading questions so that they can answer questions more independently. As long as I know which question they are having trouble with we can just have discussions over the phone. Normally I don’t do this as students are typically either available to meet in person or have access to an iPad.
So yes, after a certain point I can conduct a perfectly productive session over a phone call. So even if a student is in the mountains, as long as they have a signal, they have no excuse to fall behind with test prep!