We use Yealinks and provision using the Vital PBX and the Yealink YMCS. However, we cannot get phones to auto prov by doing the Factory Reset on the handset, which we used to do with our old PBX.
We have to Factory Reset the phone to default, log in to the GUI and then manually add the string from Vital Provisioning and do it that way.
Which is very clunky and doesn’t give us the flexibility of YMCS handset management.
I use this and it works but you have to make sure you have the provisioning URL link properly defined in the YMCS. You also have to have the https or http ports open on the Vitalpbx server at least to the WAN address where your phones are. You could also use a dhcp option 66 in the local router if needed.
I can assure you this works fine if you have it configured in the Yealink side correctly with the right URL and user/pass. This would not be anything on the PBX side if it works when you manually enter the details in the phone.
We had the same problem and what we discovered is that my folks would copy the provisioning url from vital pbx and paste it into the YMCS server field and there was a space before the url. Might check that there is no space in front of the url.