Hello @ingramatics ,
Thank you for reaching us.
Fortivoice 40 is an old device and it is out of order and it will be end of support on coming August. The replacement device is 20E.
Generally all Fortivoice devices are supporting standard SIP. So it suppose will work with Polycom and GrandStream as they are SIP standard as well.
You may try to configured the extension with Device Type "Generic" not "FortiFone".
Then you can manually configure the phones with Fortivoice SIP IP, Extension number, SIP password and port 5060 UDP/TCP.
Please don't hesitate to contact me if you have any further inquiries.