The short answer
A phone at home and one at the office
A Poly desk phone extension can carry up to three Poly phones at different places, plus the Webex apps. Every phone rings on every call, and whichever is picked up takes it. The second and third phones carry no monthly charge.
Each phone is added on the extension's Phones tab by its MAC address, which gives it its own line under the extension. That step matters: a second phone set up by copying the first phone's line details never shows up in the portal and cannot be told apart from the first. After a phone is added, both phones need a power cycle, because the first one's configuration changes from private to shared.
The new phone still needs to be pointed at our server once, the same way as any new phone: see setting up a Poly phone.
Replacing a phone
A replacement replaces. The new phone becomes the extension's primary phone and the old one is removed, so send us the MAC address of both. We keep the old phone's details on file in case it ever comes back. The new phone is then set up like any new phone.
Moving a phone that already ran here
A phone that was already on our system holds our provisioning address, so it needs no setup guide. Reconnecting it at a new desk, a new office, or on a different person's extension is one step: send us the MAC address and which extension it should ring, we attach it, and you plug it in. If it was factory reset or bought second-hand, it may hold another provider's address instead; then we re-provision it remotely from the MAC address, still without typing anything on the phone.
Cordless
A Poly Rove base is one extension, however many handsets sit on it: every handset rings and dials as that one extension. A Rove cannot be added to an extension that has a desk phone, and a desk phone cannot be added to a Rove's extension. To ring a cordless alongside a desk phone, the Rove gets its own extension, which is its own seat, and the desk extension rings it through Simultaneous Ring.
A Yealink cordless set (one base and one handset) is one extension in the same way. Multiple handsets on one base are not offered; a second cordless is a second set.
Yealink desk phones
Sharing a line across phones is a Poly feature. A Yealink extension has its one Yealink phone plus the Webex apps. Someone who needs a phone in two places is better served by a Poly on both desks, or a Yealink at one and the Webex app at the other.
Where to buy the extra phone
Through us, or from the links in the phone buying guide. Either way we need the MAC address before the phone can register.