This page Phone Apps-Supported Devices - PBX GUI - Documentation says “Sangoma P phones… are not supported by the Phone Apps module. These devices support the new generation of Apps called DPMA Apps.”
However, that very same Phone Apps module seems to be what provides DPMA Apps to the Sangoma P phones. Fixes to the Sangoma P-Phone apps require that “Phone Apps” module to be upgraded, and the Phone Apps changelog references DPMA.
So, that makes me believe that the same module handles both REST (for non Sangoma-P) and DPMA (for Sangoma-P). And, if that is the case, then the message on that website above stating that the Phone Apps module does not support Sangoma-P … is not accurate.