SIP services

Beyond making and receiving calls, you can implement a series of SIP services. These services include, but are not limited to, Call Transfers, Call Pickup, Call Hold, Call Forward, and many others. Fortunately, RFC 5359 (SIP services) defines a standard way to accomplish these tasks. Most SIP phones comply with the way SIP services are implemented; however, to make them work, you need to make sure that all the components in the network support some specific RFCs. As an example, the call transfer requires the support of the REFER method defined in RFC 3515 and the Referred-By and Replaces headers defined in RFCs 3891 and 3892, respectively. If you intend to provide PBX-like services using a SIP proxy, you have to make sure that all the components, including phones and gateways, support it. SIP services are implemented in phones, gateways, media servers, and proxies. All the components must collaborate in order to implement each specific service. The following are some of the services defined in RFC 5359:

• Call Hold
• Consultation Hold
• Music on Hold
• Transfer—Unattended
• Transfer—Attended
• Transfer—Instant Messaging
• Call Forwarding—Unconditional
• Call Forwarding—Busy
• Call Forwarding—No Answer
• 3-Way Conference—Third Party Is Added
• 3-Way Conference—Third Party Joins
• Find-Me
• Incoming Call Screening
• Outgoing Call Screening
• Call Park
• Call Pickup
• Automatic Redial
• Click to Dial

It would be counterproductive to describe in detail each service here. Refer to the specified RFC for details.