I'm looking to set up two Polycom IP 550s sharing one incoming number. Can you hold and transfer between the two phones with just basic SIP service, or do I need to get a Virtual PBX service to do that?
Capable Polycom IP phones support several methods for sharing a line registration:
- Shared Call/Line Appearance (SCA or SLA)
- Bridged Line Appearance (BLA)
Both methods are an address of record managed by a call server. The server allows multiple end points to register locations against the address of record. The difference between them is their implementation; each uses a different method of signaling in SIP to achieve the shared functionality. To that end, both use different sections of the same RFC (3265). SCA is limited to one active call on one device at a time, so you can hold then pickup a call at another station.
Similarly, later versions of Busy Lamp Field (BLF) support 'Directed Call Pickup' on a remote line, and also uses a completely different method of signaling to function. You have lots of different implementations and annoyingly similar acronyms to choose from ;)
Because of these differences, it will ultimately depend on and require specific support from the PBX that the phones will register to, to decide if you can achieve a fully functioning shared call appearance. I'd recommend researching each and searching for a PBX or similar SIP service provider that supports one of those methods. A good Polycom-specific resource to get you started would be TB33230 from the Technical Bulletin section of our website, which discusses SCA in more detail (note: this document is intended for SCA on the Broadsoft platform).
If anyone else knows of an easier/better way of implementing a shared line, we'd love to hear your input!