It's inherent in a synchronus communication medium like a phone call. With letters and email, you can respond at your leisure, but with a phone call you have to

It's inherent in a synchronus communication medium like a phone call. With letters and email, you can respond at your leisure, but with a phone call you have to make an instant decision whether to answer it or not. Seeing as people both A) don't want to be rude to their friends and co-workers and B) don't want to have their lives dictated by a ringing phone, white lies ("sorry, no service", "sorry, didn't see your call") are more or less the most reasonable solution. They keep the benefits of instantaneous access and mitigate most of the drawbacks.