My wife and I have an arrangement. If she calls me and I can't take the call, I don't answer. However, if this is an emergency, she hangs up and calls again. If she calls me twice within the span of less than a minute, I know it's an emergency, and I will drop everything I'm doing, leave in the middle of a meeting, stop in the middle of delivering a keynote, and take her call.
However, even when I do answer the phone, her first question to me is, "can you talk?" If I can't, I will say so, and we will speak later.
When Maya was a toddler, she wanted to call me at work for the first time. My wife gave her the phone, explained to her how to call me, showed her how to dial, gave her the number, but also said, "remember, as soon as dad answers, you must first ask him if he can talk right now."
When the phone rang, I was busy, but not enough to ignore it completely. I picked it up. Maya, on the other side, started with, "Daddy, can you talk?" Unfortunately, I couldn't, so I said "No." Maya went on to tell me everything that happened to her that day, everything she is excited about, and couldn't be stopped...
You see, my wife told her to ask if I was busy, but not what to do if I said I was...
Part of being trusted is having certain shared values with the other person. One of those universal shared values is that you care about me at least as much as you care about yourself. At least not a lot less than you care about yourself.
So, the next time you call me, whether it is because we are doing something together, or if you are trying to sell me something, or get me to respond to a survey, of even if you are a friend of mine, ask me first if this is a convenient time for me to speak, even though I picked up the phone voluntarily. It doesn't take long. I promise. You will show that you care about me, and therefore that we share values, and that I should trust you. It's not hard, but those first 8 seconds will set up the trust I have in you for the rest of the call.