I read it in the gmail blog that they have not provided gmail phone option to everyone, however they will send out this facility, to those who don't have it now, later. I am a regular user of google voice, and was thinking if usage of their google voice might be the reason they haven't given me the facility of gmail phone yet on my gmail. My browser are compatible, as required, to run gmail phone.
Try this link: google.com/chat/voice and if doesn't do the trick, then my guess would be that your Gmail's language is not set to English (US). If that's true then switch it to English (US) and check it again.