Is this the same phone model??
Have you tried upgrading the FW or swapping the IP Phone for other one?
Do you have anything special in the config for those 2 DNs??
Could you please delete and re-add the phones. Do "no create cnf" and then "create cnf" and reset the phone to register them again.If the issue still persists, please upgrade the firmware of the phones and check if that resolves the issue.
If the issue still persists, please collect the following logs for a test sample:
debug ephone detail mac-address <mac address>