I have a client wanting to use portable phones in a large facility. I know I can use the Engenius phones and an analog port but Engenius also makes a digital port adapter that they say works on a Avaya Definity system.
Is the Definity digital port the same as the magix digital port?

Are there any other portable phone solution that provide multiline capability, etc?

I recommend calling Engenius to be sure, but I would put money on a "No".