The iPhone allows you to create a conference call with the ability to talk with up to five different callers.
To make a conference call, perform the following steps:
1. Make your first call.
2. Tap Add Call and call another line. The first call is put on hold while you are doing this. If you want, you can talk on the second line privately before merging the calls.
3. Tap Merge Calls to bring the current line into the conference with the other caller(s).
Perform steps 2 and 3 to add more calls to the conference.
If you have an incoming call that you would like to merge into the conference, tap Hold Call + Answer, then tap Merge Calls.
Source: iPhone Users Guide

Worked on T-Mobile without any issues. 3gS iOS 4.0.
Thanks!
Have iPhone on tmobile but cannot conference call whe
n I merge calls one person stays on hold tmobile or apple can’t sort it out please someone help !!!
I had the same experience w/ conference calling not working. I called AT&T 1.800.331.0500. They refreshed the 3-way calling setting and it now works.
There’s alot of free conference call companies and telesemainar companies that offer some good services.
The capability (feature) has to provisioned in the switch, or HLR. When one isn’t able to perform those functions, something’s usually awry in the switch. (AT&T’s) side. Although I left AT&T prior to the iPhone being released, the rules, technology, and theory remain the same. I went to conference a call while my father was in Beijing for the Olympics, with my sister, and another party. I discovered from a supervisor researching my request that the old $5.99 bolt-on feature was no longer necessary to initiate, that Apple’s agreement with AT&T enabled it. Just remember, minutes are being used on each line, along with international rates.
same problem i have been experiencing i dont know how to handle this i cannot make conference on my iphone 3gs
It’s a configuration issue according to AT&T. Just (AT&T) give customer service a call and they can correct this issue. Perhaps this only happens to current customers that migrate to an iPhone where 3-way calling is not active. In any event, Roy the customer service rep switched the feature on and had me test the feature. It worked as it should per the steps above with no problem. It’s not an iPhone hardware or software issue… it’s an AT&T configuration issue.
I am unable to do call conferencing. After adding the new call, the merge function is not working. One call is going into hold mode. Please help
Good for casual calls if you can make it work. But for more important phone conference calls, it’s best (IMHO) to get a conference service and have everyone call into the same number.
I have problam with conference call with Iphone of apple.
I got problam when i go for add new call, first call is going on hold that is correct but when i tried to merge second call with first call which is on hold then it not become possible to do conferance and it shows any one call still on hold.
if somebody want to help me in this issue of IPhone Pls. reply me on my email Id i.e parikh.parag@yahoo.com
Thanks
-Parag Parikh
I also had a problem with making a conference call with my iPhone as it would simply switch between the two numbers, with one of the numbers on hold. The Apple store genius bar folks could not solve the problem. I went to the ATT store. They confirmed the problem exists, and then by modifying something in my account they got the conference feature to work. Therefore I recommend to speak to ATT about this problem.
I am unable to do call conferencing. After adding the new call, the merge function is not working. One call is going into hold mode. Please help
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