You can't send a fax to itself for the same reason you can't call the phone you are calling from.
Since fax is just a phone call, listen to it happen, as admin enable this via:
Device > Tools > App Settings > Fax App > Fax Volume and crank them all up full.
Then send a fax and relive the 90's, because you will hear the phone call and be able to verify it has.
A dial tone.
The call gets dialed
The other side picks up
Handshake occurs (Dial up internet sounds)
From there you should be able to figure out the why. There are no real settings to configure beyond enabling it (You did because you have a fax icon) and telling it what its own phone number is (And you don't actually need that), And that is all done on the screen where you enable the volume up above.
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