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Page 45 of the iPhone User Guide explains how to end the current call and answer the incoming:
Respond to a second incoming call:
• Ignore the call and send it to voicemail: Tap Ignore.
• Put the first call on hold and answer the new one: Tap Hold Call + Answer.
• End the first call and answer the new one: When using a GSM network, tap End Call + Answer.
With a CDMA network, tap End Call and when the second call rings back, tap Answer, or drag the slider if the phone is locked.
I hope this information helps ....
Have a great day!