Honestly, I get so few phone calls, that I just pick them up straight away. ._.; Basically if I give excuses, it's really why I didn't pick up the phone.
I love texts, so I usually reply to those, but if I miss some it's because, well, I don't notice them and forget to reply. So that's what I'll tell them. I've probably intentionally not replied to a few but people didn't notice.
If I make excuses for not replying to people, it's over IM... 8) *whistles*