I had trouble with my 3 branded phone connecting to my work and home Trixbox (Asteriek) servers initially as well. Using the exact same setting I got it to work by updating to the latest firmware.
I also had to change my phone's product code because E71's that have 3 product codes don't get the very latest firmware. It might also be that 3 firmware (and other providers maybe) blocks VOIP, I don't know since the firmware versions I tested weren't the same. All I know is the latest generic firmware got it working for me.
You'll have to google for how change your phone's product code (it's easy) and find an appropriate product code(ie. Australian product code for either E71-1, E71-2 or E71-3 depending on what you have).
Once the product code is updated you then just use Nokia's own Software Update program to download and install the new firmware.
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