Airtime is used from the moment you press SEND to the moment you press
END or the phone disconnects. It makes no difference if the called
number answers.

The clock starts when you press the Spend key.  There are some
free calls, like dialing #MIN# (on the postpaid service).
For incoming calls, I guess the timing starts when the call hits
T-Mobile's switch (roughly, when your phone starts ringing, if
it's in range).  But you still get charged if your phone is off.

To test this, don't use your phone for about an hour, then dial
#MIN#, and note how many minutes you have used.  Then make a phone
call (or receive one).  Wait about an hour and dial #MIN# again.
Be sure that the "as of" date is after you made the call.  Subtract
the two and see how much you got charged for this call.  This works
better if you have only one phone on an account, or you can get
everyone to cooperate by not using their phones during the test.
Do teenagers stop talking on the phone around 4AM on a Monday?

My Motorola RAZR keeps track of the time used, incorrectly.  The
phone tries to measure "talk time":  the time between making or
answering the call and hanging up.  That's not how you are billed.
The time shown on the phone may be too LOW because:
- The phone starts timing when the call answers, but the billing starts
 when you press the Spend key.  That's at least 1 minute too low for
 every call that doesn't answer or that you don't answer.
- The phone keeps the talk time in seconds, but billing rounds up to minutes.
 1 minute, 1 second gets billed as 2 minutes.
 That's an average of 30 seconds too low per call you answer or that answers.
- Calls made to you when the phone is off or out of range still cost minutes.

The time shown on the phone may be too HIGH because:
- The phone counts minutes during times when you've got unlimited talk time
 that doesn't use minutes (for my plan, weekends).  This might also apply
 to Mobile-to-Mobile, or calling Voice Mail, or calling Customer Service
 depending on your plan.
- You didn't reset the counters at the exact time when the minutes get
 refilled.

According to T-Mobile's "Terms and Conditions":

       We round up any fraction of a minute to the next full minute.
       ... Airtime usage is measured from the time the network begins
       to process the call (before the phone rings or the call is
       answered) through its termination of the call (after you hang
       up).

http://www.t-mobile.com/Templates/Popup.aspx?PAsset=Ftr_Ftr_TermsAndConditions

This doesn't address the case where the call isn't answered.