My experience is that when you call right on time, people are often trying to finish up that "last little thing" before taking the call -- the last phone call, little meeting, run to the bathroon, etc., and they aren't quite ready for the call. You'll often catch voice mail. Give them the 2-3 minute buffer, and many people have that "last little thing" cleared from their plate, and they even have a minute or so to prepare for your conversation. This helps the productivity of the call, as well as helping to avoid a little game of "phone tag."   Try not to call more than five minutes late, though, as you will likely be perceived as "being late". It is even possible the person you are calling may think you've forgotten about the call, and will leave their desk to start something else -- this obviously defeats the purpose. With planned phone calls, 2-3 minutes late is great, more than 5 minutes is late. Try this strategy, and let me know if you also see it works for you.