How often have you found yourself calling a friend to get the phone number of a mutual friend? And then having to hold the phone while your friend pulls up the contact list on their phone, then recites the number to you, and then you write it on paper because your phone won't let you add contacts while you're on a call, and then you misplace the number you wrote on paper, ad nauseum. Why isn't there a single button that says "Send this Phone Number to the Current Caller"?
It's a common problem. You're out and about, and realize you need to call a specific person, but you don't have their phone number (or more often, you have it on your desktop computer, or your laptop, but that doesn't do you any good in the current situation.) So you decide that the best thing to do is to call a mutual friend and ask them.
When they receive a phone call from you, they're fumbling to hold the call while they look in their address book. (That is, if they're lucky, and if their phone actually lets them open the contact list while they're on a call.) More often, what happens is that they tell you to hang up while they consult their address book. And then you have to hunt for a piece of scrap paper because your phone won't let you add a number to the list like that.
What the world needs is a button next to each phone number in the contact list that only appears whenever you're on a call. The button, when pressed, sends an SMS from you to the current caller, and contains within it the information from the contact record you just selected. It doesn't have to be too fancy, a two-line VCF record should do nicely.
If the recipient's phone understands this method of contact transfer, it can prompt the user and import it automatically. If not, the user can still read the SMS herself, and dial the number. No more paper, no more fumbling, no more "let me call you back".
It's so easy, a caveman could do it. If only phones implemented it!