I have not been able to get the conference call feature to function properly. For example you call phone a and establish a call, then you try and bridge in phone b. Phone b doesn't answer and goes to voice mail. On the 9900 you simply went back to phone a with the green button and it dropped phone b that didn't answer. I am told I should see the following screens:
Click on the 3 line icon above the caller's pictures
I had a similar problem with conference call where I couldn't connect to a third line. After going through every single setting on my phone and not having any luck, I called Vodafone UK and told them of my problem.
They said they had to enable conference calling on my line, which they did, then told me to wait a couple minutes and try it again. It worked after that.
So try calling your network provider. It may be that they disabled it for some reason.
Opened a case with Verizon. They tested the feature with me and verified it does not work. They tested the feature on a Q10 running 10.1 in their office and it did the same thing, does not work, does not allow you to drop one of the parties.
Found the answer to my problem. It is crummy Verizon!!!!! Their network will not support the feature:
"Thanks to some guinea pigs, I mean friends and family, I tested on CDMA and GSM wireless providers. However, conference calling on CDMA differs from GSM (much to the dismay of yours truly while putting together this 101). The good news is these features work on GSM. Bad news is they are limited or unavailable on CDMA. For example, VZW customers can't drop calls or join one call on hold with one currently in progress like AT&T. On the bright side, they do let you use the green send key to join calls."---I found it here: BlackBerry 101: Conference Calls and Three Way Calling | CrackBerry.com
Unfortunately I do not make use of this feature. 
