Comcast's phone service does not provide you with SIP credentials. You can't use it on anything other than one of their approved cable modems or MTAs. You can't use the 2182.
In theory, you could port out your Comcast Xfinity phone number to some other service provider. However, if you intend to remain a Comcast customer, the incremental cost for their phone service is less than you'd pay elsewhere for the same level of service.