Ensure that you notice the waiting call or sms if you are already on the line.
You can configure the phone to vibrate, play sounds, adjust notifications per call, and adjust the sound volume when a call or sms is waiting to be answered.
One-click sms response to send a text message to missed or waiting callers.
Send customized one-click sms responses to calls you don't answer.
This is also useful if you are already on a call, and another call is coming in.
NOTE: Security restrictions on the Droid block in-call notifications until your calls have ended.
Also see Call Waiting Configurations to avoid missing calls & sms messages