I have the same problem.
My phone is an MC603C/A which from what I can tell is the 4s. Siri (or voice control) does appear when I hold down the home button but, when I say "text john" Siri defaults to facetime. Also, when I try to check the settings, Siri does not appear in my settings anywhere, nor does it show up in the list of restrictions. When I ask "what time is it?" the response is appropriate. Any ideas?