We have currently trouble with Phone feature registration on two devices: DX70 and DX80. DX650 registered successfully in the same network with same TFTP server (10.10.20.1).
Looks like, that something should be triggered by server side because there's a working connection from this devices to 10.10.20.1, but phone service is not available anyway.
We've tried to config it via Shared Lab, but maximum result that was acquired was that DX80 was somehow set to Phone only mode (which is another issue).
Running config of router attached.