In the US, cell phone advertising is largely carrier-based. The cell phone manufacturers do some advertising on their own, but it's mostly the carriers that buy the TV spots and decide what phones to advertise. In the context of the general subsidization model in the US, this advertising model is considered a "win-win" by the carriers and manufacturers: the carriers want to advertise the latest and greatest models in order to drive new subscribers and retain existing contract holders, while the manufacturers get free (or highly subsidized) advertising.

Apple is the exception to the rule, they produce and buy their own spots, I believe.

And finally, unlocked phones aren't usually advertised on TV. Again, the manufacturers are used to not having to pay for a lot of the TV spots advertising their phones, since the carriers are picking up the tab. They probably are also hesitant to go out on a limb and spend what it takes to build an effective TV campaign around a phone without carrier support.