3 way calling not working

After reading all your posts on the issue, it may just be since this is a user to user forum, no one else has figured out the problem either and I assume you've tried the chat and other avenues.  I'm not a u-verse user, but from what I've found the 3way feature is the same for u-verse as POTS, so here's a suggestion.  The phone companies use calling party control on POTS service so the call disconnects after so many milliseconds after seeing the CPC signal, so I suppose it's possible that the CPC in the toll switch, or whatever they call it on VOIP, is quicker than on the local and the flash button is too slow so it sees it as a disconnect instead of a flash.  So, if you haven't tried this, instead of using flash do it manually, as quickly as you can.  The sugguestion to use a different phone is not all bad as the flash rate is different between brands.   It's at least something you can try.  This is definitely an AT&T problem, so that's who will have to fix it, convincing them it's their problem is another issue. 

Scholar, Thanks for your reply but I don't understand what you meant when you said "instead of using flash, do it manually"  what am I suppose to do manually?

Use the switch hook or phone/talk button if it's a cordless phone.  If you have a corded phone with a switch hook I'd try that first, but a cordless phone/talk button works similar to  a switch hook  flash.  

I will try this but again it doesn't make sense that 3 way calling works Local to Local by following the exact instructions using the flash/hold button but does not work for Local in FL to Long distance in TX

The reason is as I explained, there is probably a difference in the timing of the CPC between the local and long distance switches (server or whatever it is on VOIP, I know it's no longer a physical switch) and this will be an AT&T problem to correct.