I found this explanation on the net.
The main cause of this whole act of Samara was to get heard. By whom!! By everyone. She was neglected her whole life. Nobody loves her. Every close person (her adoptive parents and her biological mother) can sense evil in her, so they avoided her. But she wanted to get noticed. The reason behind those videos and killings was to tell everyone that she existed and how her adoptive mother killed her.
So to spread her story she wanted that video to get spread. That is why she is giving people options. Option 1: spread the video, copy it, show it to others (we can see it in the end when Naomi figured it out); Option 2: If you failed to spread it you'll die. And Samara allowed them only 7 days as she was alive in the well for 7 days after her mother pushed her. So the phone ring is something like "your countdown begins", basically to inform them that you have to do something after watching this video. Samara has no other way to interact with them.
The explanation was based on the English version.
[(My theory: Samara was a very tech-loving evil. She used video to kill people. She used TV to scare them. That's why she chose phone, another machine, to spread fear. :) Just kidding]