The technical bit
For direct access, you need a Private Branch Exchange (PBX) that supports the Q931 signalling protocol.
Alternatively, if you want to indirectly link your PBXs or small office/home sites to EE, then you'll need a BT exchange line (e.g. ISDN30 or standard DEL) to each of these sites.
We can arrange for one of our Technical Account Managers to visit you to discuss the best solution for your business. Just get in touch.