IMO it’s not a good idea to run any other service on a FreePBX system. Find some way to keep them separate.
If your trunk is SIP, consider setting up FreePBX on an inexpensive cloud VPS. For small systems, that is generally more robust. It continues to operate during an internet or power outage, taking voicemails or sending calls to your mobile. If the hardware fails, you’ll get a replacement automatically. If the software has a major issue, it can be quickly restored from a snapshot. The cloud service will probably cost less than the electricity consumed by an old desktop.
If you need an on-site system because your trunk is an analog line or VoIP from your ISP, consider running it in a virtual machine on a capable server that runs continuously, or a small processor such as Raspberry Pi.
Alternatively, find another home for your development system. You could take advantage of the free trials offered by Amazon, Google, etc. Or, use your own PC for development (if it’s Windows or Mac and you want a Linux development environment, set up a virtual guest machine).