Adding non predefines/listed SIP provider trunk
For now, I simply selected one of the providers from the dropdown list (voipvoice sip) and filled out bogus info and saved.
Then I proceeded to FreePBX (Admin → Advanced → Connectivity → Trunks)
and select the listed bogus trunk for editing and filled out all the details with the info from my local provider. That worked for me (registered and can receive calls), but a generic SIP provider empty template as a dropdown list option would be much nicer.
ps. When trying to add the trunk via FreePBX, and thus skipping NethVoice GUI, will lead to a partially saved truk in NethVoice that can not be edited, nor saved. So hence first a bogus trunk via NethVoice and then correcting it in FreePBX is the only way to add a SIP trunk from a provider not listed.
Trying to duplicate a trunk in FreePBX created via the above workaround will result in the same partially saved duplicate trunk that will fail. So creating a new trunk as described above is the only way to get a duplicate.
HTH