Since I'm at home, I looked at my SPA504G's settings..
Yeah.. Server register (SIP server hostname).. but that blah.simonics.com there.
Username becomes the Authenticaion ID.
Secret is your Authentication Password.
As for External Number.. unclear if you need the GV1##########, 1##########, or just ##########, I don't have it hooked into a PBX at present, so this bit doesn't apply to my handset's registration. You can sort that one out.
Based on the "Registration string" they provide.. my guess is the External Number should be 1##########.
That should be sufficient to get the trunk registered. As Leejor states, you will need an outbound dialing rule established if you want to test outbound. For inbound, you also have to tweak an option in Google Voice.. don't ask me which one.. perhaps Chat?.. would only affect inbound call routing with regards to Google. Test.. it should route the call to 3CX. If not.. check the settings in your GV account. I believe you will "not" want call confirmation set, for starters.. but that may not be a problem after all (unless you have an IVR answer..

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