how to place a conference call on iPhone 6

Why can I not Marge a call from my iPhone6?

http://www.dummies.com/how-to/content/how-to-make-a-conference-call-on-your-ipho ne0.html

Here are the steps to follow in order to conference with up to 5 people:

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 person. Tap Merge Calls to resume the conference.
• Add an incoming caller: Tap Hold Call + Answer, then tap Merge Calls.

Steps in iPhone iOS 9 Manual:

https://help.apple.com/iphone/9/#/iph3c99551d

I am trying to make conference call my phone enables me to add caller and swipe from one to the other but cannot add caller to do conference call

How to conference call