for iOS 9.1 software

Put the first call on hold and answer the new one: Tap Hold +
Accept.
End the first call and answer the new one: When using a GSM
network, tap End + Accept. With a CDMA network, tap End and
when the second call rings back, tap Accept, or drag the slider if
iPhone is locked.
With a call on hold, tap Swap to switch between calls, or tap Merge
Calls to talk with both parties at once. See .
Note: With CDMA, you can’t switch between calls if the second call
was outgoing, but you can merge the calls. You can’t merge calls if the
second call was incoming. If you end the second call or the merged call,
both calls are terminated.
Conference calls
With GSM, you can set up a conference call with up to five people
(depending on your carrier).
Note: Conference calls may not be available if your call is using VoLTE
(Voice over LTE).
Create a conference call. While on a call, tap Add Call, make another
call, then tap Merge Calls. Repeat to add more people to the conference.
Drop one person: Tap next to a person, then tap End.
Talk privately with one person: Tap , then tap Private next to the
Conference calls