I sometime find myself in project manager's shoes and need to remind people of duties they have to carry out for assigned projects. I do not want to be too pushy but after a few emails I tend to call them. If they do not pick up quickly I wait a couple rings and then hang up, send a reminder email, and eventually go see them in person. You get the idea.
Let's focus on the phone part. The phones do not have voicemail. As a rule of thumb, I wait for 20 seconds for people to pick up in order not to disturb all the people in their vicinity with the tone. My assumption being, 20 seconds are sufficient for someone to pick up a fixed line, if they are available (the same wait time would probably also work for mobiles).
Is that a reasonable practice? How long should I keep the phone ringing?