I think you nailed it - phone call today is far more important and intimate than it ever was. Same thing with an SMS - it's a guaranteed read. To me, if it's important - send me an email or a letter. If it's urgent and super important - send me an SMS. If all of my networth is going down the drain and you need a split second decision, call me.

And at least from my perspective, cell phones are only making this truer given that you get interrupted wherever you are and whatever you're doing. In my case, that means I basically just give my cell phone number to people I want to be able to reach me. Everyone else gets a work or home landline number. And good luck reaching me on one of those--especially if I don't recognize the caller ID. I don't even check messages when traveling. (I do still maintain a landline primarily because I sometimes need to make high quality calls for business purposes--and other options aren't as reliable.)