OnPrem PBX

We are currently using a Grandstream PBX. We switched to it after using an old digital Panasonic system. Our ISP doesn't support SIP, so our lines coming in are still copper, and we can't port the numbers out.

We are building a new office and management has told me to find something else, I have experience with FreePBX and 3CX, but I don't think that will be well received if they don't like the Grandstream.

Any recommendations that will work with copper lines?