The t29 is a relatively late issue phone and its template is found in V14. It is apparently not included in prior versions (even after looking at the updates); hence why it may be a good idea to maintain a maintenance contract in order to keep the system and the available phones abreast of one another.
The provisioning file has other models in it as well, but the T29 is there as I just used them last month in a hotel. While I see no reason that the file will not work in an earlier version, it may cause some issues of which I am not aware.
If you have a reseller or a support contract, you should contact either and see what they can do to resolve. I do not know that 3CX would take kindly of me providing the file as I assume that if they wanted earlier versions to have, they would have made it available.
Others may have developed their own version that they would be willing to share.