This can be true to a certain extent, but after awhile can be used as a convenient excuse. Almost everybody can take five or ten minutes out of their day to answer a message, but on the other hand if the person that leaves messages becomes a pest leaving them too often, then I can certainly understand why their messages might get ignored.

I try to return business calls or emails within 2 hours of receipt, 24 at the most.

Even if that return is to say only that I received the message and I'll get back with more details later.

"Busy" is a reason for delay, not for never returning a call.

Best reason there is. In fact it's dealing with large volumes of e-mails, e-messages, voicemails, and handwritten messages that will make you busy. Time constraints can't be denied. The old rules of etiquette required answering all communications - and I sympathize with that view - but it just isn't possible given the reality of the situation today. Hopefully the senders understand that.

Legitimate reason, some times you have to ignore messages to get something done, especially when your on a deadline or trying to leave work.

Depends on what one calls "busy". Actually, if one isn't careful, using this line can become an excuse, even for one who really is busy. I'm busy, yet I try to work it into my schedule to tend to my messages, be it phone, E-mail, or whatever. I believe it is only right.