Conference Call Feature

I am a newbie to FreePBX and have read as much documentation as my brain can take at the moment. But we have a requirement to have inbound calls to an internal extension. While on the phone we need the ability to conference in another person either by extension or by external phone number. I don't see this functionality in the documentation but I could be missing something.
Core 2.5.1.6 is the version we are running at the moment. I don't know if there is another module that I need to install.
Any support would be appreciated.
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If you are using...
IP phones, most (even single line) have the ability to conference in another party by using the phone's conf or conference button. The SIP (VOIP) phne takes car eof this for you. I have several Grandstream and an Aastra phone and all work as they should.
If you are using analog (Zap/DAHDI) phones, then you will have to set up a conference room, give it an extension and transfer your callers into the conference room. The you join. This is of course if you have single line analog phones. Some multiple line analog phones can conference in a third party.
John