Comcast just launched a new phone service called Xfinity Mobile

Comcast on Thursday took the wraps off of its new mobile phone service, Xfinity Mobile.

The company first confirmed it would launch a wireless phone service to compete against the likes of Verizon and AT&T last September.

Xfinity Mobile won’t be a complete rival to those, though: It’s an MVNO, or mobile virtual network operator, that will piggyback off of Verizon’s wireless towers. This is similar to services like Cricket Wireless, which uses AT&T’s network, or Ting, which uses T-Mobile and Sprint.

However, Comcast says it will bolster that network with its 16 million WiFi hotspots around the country. This is along the lines of what Google does with its Project Fi service. You’ll be automatically connected to one of those by default, then get kicked onto Verizon’s network when you’re out of range. Comcast says it will authenticate connections to those hotspots automatically.

Comcast has been able to use Verizon’s mobile network since 2012, when it sold a chunk of wireless spectrum to the mobile carrier. This puts Verizon in the driver’s seat, to an extent, since it could theoretically raise the rates for using its network if Xfinity Mobile ever eats into its mobile business. That’s part of why Comcast is emphasizing this as a WiFi-first service.

In any case, the big wrinkle here is that you need to already be a Comcast subscriber to use Xfinity Mobile. Comcast openly says it wants to use the service to help drive cable subscriptions, and is only offering it to those who subscribe to an Xfinity internet package. The idea is to make it so, like Verizon, it can run every internet connection you may use, and get more revenue out of the subscribers it already has.

Here’s how the pricing breaks down:

• If you subscribe to one of Comcast’s higher-end X1 internet and cable packages, you can get an “unlimited” plan for $45 a month per line. Comcast says roughly 25% of its X1 user base would be eligible for this today.
• If you subscribe to any Comcast internet service below that, you can get an “unlimited” plan for $65 a month per line.
• If anyone on your plan thinks they'll only use a small amount of mobile data per month, they can pay “by the gig,” in which each GB cost $12. Comcast rounds this up, so if you use 4.2GB in a month, that counts as 5GB, and the line costs $60 that month.
• Every plan comes with unlimited talk and text.
• All of this is done through auto-pay billing. 

In any case, Comcast says the first five lines on an Xfinity Mobile plan will have no access fees. It will charge taxes, though, so these rates will be a bit higher than advertised.

The company says you can mix-and-match with multiple line plans: A less-data-hungry user could use the $12/GB plan, while a heavier user could buy an "unlimited" plan. You can also switch from the $12/GB plan to the "unlimited" plan midway through a given month if you find yourself eating up too much data.

On their own, Comcast’s unlimited plans are less expensive than similar offerings from T-Mobile, Verizon, and AT&T, which range from $70 a month to $90 a month to start. Those plans do not require you to subscribe to another cable or internet service, though.

And as with those others, Xfinity Mobile's plans aren't really unlimited: Comcast says it will reduce speeds after you use more than 20GB of mobile data. It will set them at 1.5Mbps until the end of the month, too, not just temporarily slow them in areas of congestion, as is the case with the other unlimited plans.

Comcast says it will not limit mobile video to a less-than-HD resolution, though, as others have in the past. It won't put any special restrictions on mobile tethering usage, either.

In terms of device support, Comcast mentioned that it will offer the latest iPhones (7, 7 Plus, 6S, 6S Plus, and SE) and Samsung phones (Galaxy S8/S8+, and Galaxy S7/S7 Edge), along with LG’s mid-range X Power phone.

You will not, however, be able to bring your own device to the service to start. Comcast says it will add that option sometime in the future. There's also no WiFi calling support, which other networks use for clearer audio. There will be an option to reach customer support from an actual person through text messages, though.

Also of note: Comcast says it will not exempt its own video services from taking up data over Xfinity Mobile. That practice, known as "zero-rating," has been used by various other mobile carriers in recent years, and has raised concerns from net-neutrality advocates along the way.

Comcast did not give a specific date for when Xfinity Mobile will be available to consumers. We’ll have to test the service to see how well its mix-and-match of WiFi and Verizon LTE actually works, but Project Fi has had some success with that concept before. Either way, Comcast will be hoping the service can counter the ongoing decline of pay-TV, and give its customers more incentive to stay locked into its various subscriptions.

Watch: Comcast hit with the biggest fine ever given to a cable operator for overcharging customers

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