Got SIP response 480 "Temporarily not available" back from 10.124.193.26:5060
That one is the important log line on the 2nd call. Is your phone that returns 480 not available. Asterisk attempts to dial that phone and the phone returns that error.
FOP2 controls Asterisk, not your phone. It can add/remove astdb entries or custom devstates, it cannot and will not control your phone.
If your phone decides to reject a call, reason lies in your own phone. I do not know how are you activating DND from your own phone, but it seems it is not only dialing the FreePBX feature code but doing something else internally. Unfortunately FOP2 cannot modify your phone internals.
Look at whatever you use in the phone to set DND, as it seems it is not only dialing the feature code in freepbx but also setting DND in the device itself.