Voip to Siemens HiPath 3300

We currently have two ISDN lines coming into the office (so four lines in total) with 10 direct dial numbers. Our office is setup with a Siemens HiPath 3300. I am looking to switch over to VOIP porting two numbers with us and perhaps direct dial for staff, but wish to keep the HiPath. We have a Grandstream device (not setup) that enables VOIP connection via the analogue card to the HiPath.

There are a host of folk trying to sell their systems, many overpriced and the call charges unrealistic. I did like this (https://www.youtube.com/watch?v=8QyFidVcoLM&ab_channel=NetworkChuck) and the idea we could host on the Raspberry Pi and use our main server as a back up ... but that perhaps shows my naivety more than anything else when it comes to such things.