Is this certain?
The Fairphone 3 can do 4G and VoLTE (4G voice calling, if the provider provisioned it for the user), and this 000 calls via 3G seems to apply to phones which canāt do VoLTE, so just wondering.
This was given for a Fairphone 5, though.
On a Fairphone 3 dial *#*#4636#*#*
for the āTestingā menu, choose āPhone informationā, then have a look whether āVoLTE provisionedā is enabled. If not, call your provider to check whether they can provision VoLTE for you.