Kevnich2,
I figured my "lazy" comment would stir the pot some. I admit to being pretty frustrated with some of the views being expressed and it came out a little harsh... perhaps. But for those who are unfamiliar with the steps needed to add a second account to a phone, take a look at the many 3CX blog examples of how to configure a phone manually. But rather than performing these steps on the first account within the phone, apply the steps on the second account within the phone. You really only need the IP of the PBX, the desired extension number (ID) and phone password (listed under the extension "Authentication" section in the 3CX console). Obviously your physical phone must support at least two accounts, but I don't think there is a supported phone sold today that doesn't support multiple accounts.
Effectively what you are doing is instructing your phone up to register to two extensions. The first extension (account) is auto-provisioned through the normal provisioning process. You don't need to change the standard provisioning process. The second virtual extension (account) is configured manually under the second account. The necessary configuration steps are performed using the web interface of the phone. Naturally the user interface is different from one phone manufacturer to another so I can't offer the precise steps, but it is easy. My expectation is that once you identify where to make the changes on the first phone's settings page, you should be able to setup the remaining phones in about 2-3 minutes each. Granted, if you have hundreds of phones to configure ... that's going to take some time. In the case of large numbers of phones needing to be configured, it may make sense to invest some time building a custom template.
Regarding custom templates, again just because custom templates are not "supported" that doesn't mean you shouldn't or can't do it. That said, you can't expect 3CX Support to go through all your customizations and look for errors. There simply are too many customers around the world to support that level of effort from 3CX support. However, unlike other "unsupported" things you shouldn't do with 3CX, custom templates are there specifically to allow you to make some changes when needed. Just remember to make a COPY of the original template, and apply your changes to the copy rather than the original template that came from 3CX.
There are lots of instructions on the proper way of managing custom templates on the 3CX site. Keep in mind that during the "development" stage of building your custom template, because you are working with a copy of the original template, you won't impact any of your other phones but the one you are using for test. If you screw up the template, you can always start over by making a second copy of the original. And if you temporarily screw up one phone, it's pretty dang easy to just reapply the default template and factory reset the phone. This is not a risky endeavor.
If you are experiencing unexpected behavior after applying your custom template, and 3CX wants you to revert to the default template, that's one setting and a reboot of the phone. Let 3CX support fix your issue, then put back the custom template. If the issue returns, it's a safe bet you didn't get your template quite right. It is true, however, that if you use a perfectly crafted template, you will still see the red flag on the 3CX Management Console, but you know why it is there and you can ignore it. I personally think it should be a warning at the most (rather than a red flag, but that's just my opinion).
Maybe this all sounds really messy, but in truth this is vary straightforward and should only take a couple of minutes to setup the first phone. Templates are only really worth the effort if you are dealing with a very large number of phones. This is basic 101 level stuff.
