A license to modify source code is granted to holders of PBXes PRO.
So as a premium you can look at it, but don't change anything.
I don't know but you can set pro for free for 14 days, make all the changes and than after 14 days return to premium again.
That said I don't know if you change back that the mods you made will be kept. Also if you want to mod again you will have to change to pro again.
I understand that it's possible for PBXes PRO user, but if I'm not running a call centre or using CRM, then a premium account is all I need (especially for a home user).
Can the admin please confirm if the 14-day trial method would work? Is there any chance that the source will be open for edit for premium users as well?