My office has 10 phones, 8x VVX201, 1x VVX311, and 1x Sound Station IP5000. We currently pay our phone company for 10 lines, one for each phone, even though we never use our dedicated lines and we're all in the same location. We would like to use only 4 lines, shared between 10 phones, and use each device as an extension, not it's own dedicated line. Is there a Polycom or Obihai device that can assign the extensions and allow us to share the 4 lines?
Usually a company should charge you is for a line and extensions, but it depends on the company services that you have and the PBX type.
To have a basic solution, what you may need is a In-House SIP-PBX. Something that works in the middle to register to the upstream lines (4 in this case) and allow your phones to register to it, to have internal extensions.
There are a lot of appliances (like the GS UCM6204 / Openvox / Rhino / Sangoma) that have that kind of functions, but requires setup and knowledge of what you are doing. I'd suggest to contact your current Provider and ask for that exact setup. They may offer you a different plan that may save you money and time.