OBiTALK devices, by themselves, are just boxes of hardware. To use one, you need to pay a service provider, just like you'd pay any telephone service provider. Polycom/Obihai is not a service provider itself.
Number porting is a FCC-regulated, standardized procedure in the US. There are two carriers involved; the losing carrier (Optimum in this case) and the gaining carrier (whichever carrier you select to use instead.)
Briefly: you find a service provider you like. You contact that service provider, and ask them if they can port in your current phone number (now controlled by Optimum). If they can, then they will act as your agent (with your legal permission) to contact the losing carrier. They'll say, roughly, "Mooch wants us to take over service on one of your numbers. Here is Mooch's account number and PIN on your service, to serve as permission. Do you approve?"
Optimum then replies "yes, we approve; take it away." The gaining carrier then works with the US agency that administers all number ports, to take over service. A date is negotiated between the two carriers. On that date/time, the inbound call routing for your number is switched over to your gaining carrier. Finally, this will automatically trigger your losing carrier (Optimum) to cancel service on that number.
Do NOT cancel service yourself; you will lose the right to port the number. Keep paying for the service until the porting process shuts off your service at Optimum, after moving it to your new service provider.
Many of us here on the forum use one or more of these SIP VoIP service providers, who have long track records of reliable service:
callcentric.com voip.ms, or phonepower.