Unless something has changed since I left Sangoma, Sangoma Phones only support the integrated VPN from the FreePBX Distro - and require the SysAdmin Module to configure. @lgaetz can probably confirm if this has changed.
ClearlyIP Phones however can be configured with “generic” openVPN servers see instruction here https://kb.clearlyip.com/phones/OpenVPN-Setup.html
On RasPBX you should be able to follow these instructions to install OpenVPN manually: https://raspberrytips.com/install-openvpn-raspberry-pi/
Although https://www.pivpn.io/ is really the easiest way to install on a Pi.
The closest model to the S-305 would be the CIP-230 the MSRP is about the same, however, the CIP-230 does have a color screen and Gigabit Network Support.