I spent a lot of time trying to install SIP on the Cisco 7940 phone, it comes with something called SCCP (or Skinny Call Control Protocol) installed as default. However you can (with some effort as I found!) install SIP and connect to a SIP network, such as Sipgate or Voipfone. I scoured the Internet and found bits and bobs on various sites, but none of them really explained EXACTLY how to do it as a process. Below you’ll see steps of how to do just that, along with a nice zip file for the config & a few ringtones.