I have connected Grandstream GXP 1625 to 3CX. It works properly but in the Phones, is shows as not supported. The firmware is upgraded. Please see the screenshots below.
The firmware is versioned accordingly for our supported phones so if you current PBX edition shows it as not supported, we would recommend to install the correct version that we have tested along with the manufacturer to ensure maximum compatibility.
Reset and reprovision the phone afterwards to make sure nothing invalid is saved in phone memory.