When to Use SIP
SIP is the protocol of choice when you want to integrate multiple types of media. For example, perhaps you get an instant message on your phone screen that you want to respond to verbally. You start to press the button to call the IM sender, but you see that the sender has set his status to "Don't call me," so you refrain. Later, you would like to have a videoconference with three other people, but you notice that one of them has his phone off-hook. The other two join you, one as video and the other as audio only. You send a text message to the IP phone of the third person, asking him to join when he can. This might sound like the office of the future, but it is becoming reality.
You can use SIP trunks even without native SIP phones. You might choose them because of the simple session setup mechanism for SIP. For instance, you can use a SIP trunk between gateways with toll bypass. When you use SIP with CallManager or CME, you are not limited to Cisco-brand IP phones. You can also use other commercially available, and perhaps lower-priced, SIP phones. Be aware that CallManager might not be able to provide all the features for these phones that it can for a Cisco-brand SIP phone.
Before you implement a SIP network, plan its integration into your existing network. Determine exactly what you want to accomplish and how you will accomplish it. Remember also that you need to either find people who are knowledgeable about SIP or train your staff.