How to deal with simultaneous communication

I unexpectedly ran into a colleague of mine at the airport today, on a lay-over.  He offered me a ride home so I called my friend who was picking me up to let her know he would be driving me.

While I was telling her about running in to him, and how I know him, he was, of course, sitting right there.  When I said we were catching up on gossip he laughed and said something so I smiled and started talking more about him for his benefit.

I was having a two-way conversation, but my interlocutors were having one-way conversations.  This used to be something that only happened at home if you were talking in a high-traffic area, or if it was a long distance call to a relative, so everyone was crowded around taking turns speaking. Yes, I remember those days. . .

Simultaneous communication seems to go on all the time now. . . it can be difficult to handle.  I think a rule of thumb is to be ready to either: a) totally engage with the interrupting communique, b) intercept the communique and let them know you'll call them (text them, IM them, etc) back, c) not respond.  Each one can be done in a socially acceptable, unoffensive manner.  "Being ready" for each one means finding your method and your policy for each one. For example, my policy for phone interruptions is that I give the person I'm with "in real life" precedence.  This means, I usually do not answer the phone when I am with someone.  If I do, I let them know I can't talk and ask them a good time to call them back.

To refine even further,  how much do you go by what YOU think is appropriate and socially acceptable, vs. what others think?

Here are some factors to consider:

• emotional bidding - as long as you respond to their bid, the chances are they will be fine with whatever you choose. 
• using direct language or a firm tone - actually, being apologetic can be insulting, implying they were living for this  phone conversation with you, or like it's a favor to them.  
• politesse - "please" and "thank you" go a looong way, also showing appreciation for the person's  communicating with you.  
• intent - a lot of what people receive has to do with the intent behind our words, that overrides all of the above. 

I would like to know what the consensus is on the above issues - would love to hear your comments.