One extension can have up to two SIP devices configured to it. "Devices" include IP (or SIP) desk phones , softphones, multi-line configurations, and expansion models.  Attempting to configure a new phone to an extension that has two SIP devices registered to it will fail.

Use case examples:

• If an employee works from home or another location a few days a week, he/she may need a IP desk phone at their primary location and another SIP deskphone in their alternate office.
• Employees who travel often may have a IP phone as their office desk phone and may use a PC softphone to make calls on the road. PC soft phones are considered a "SIP device"
• Employees who utilize a mobile softphone, like Acrobits, Fring or Bria, as well as a SIP desk phone will use this option. MObile softphones are considered a second "SIP device"
• An employee has lines 1 and 2 on their phone assigned to his/her own extension while lines 3 and 4 are set up ring when another extension is called. Think of a secretary who may have his/her boss' extension ring on the phone.