Comcast is starting to make it easier and cheaper to sign up for Xfinity Mobile, its new mobile phone service, by allowing people to bring their existing phones to the network instead of requiring them to buy new ones. At first, this only applies to iPhones, but Comcast says it expects to add phones from other companies later this year.

Unfortunately, there are still a lot of limitations. You’ll have to have a somewhat recent iPhone — support runs from the iPhone 5 up through the iPhone X — and even then, not all of those iPhones are compatible. Because Xfinity Mobile piggybacks on Verizon’s network, the phone has to be one that can connect to Verizon. If you’re bringing a Verizon or fully unlocked iPhone, it should be fine. Newer iPhones bought through Sprint work, too. But most iPhones bought through T-Mobile and AT&T won’t work, so you’re just out of luck.

That means there are a whole lot of potential customers who won’t be able to take advantage of this new offer. That limitation isn’t unique to Xfinity Mobile; but since Comcast is basically starting from scratch here, it would benefit from removing any additional barriers to switching services. Comcast says it’ll offer trade-ins for iPhones that aren’t compatible, but that means you’ll still have to set up and partially pay for a new device.

Xfinity Mobile started to launch last May, and by August was available to all Comcast customers. The service is available only to Comcast’s existing internet subscribers, in part because it’s meant to rely heavily on the company’s network of Wi-Fi hot spots. That’s supposed to keep customers from using cellular data as much as possible, because all cellular usage occurs through Verizon — something Comcast then has to pay for. Plans are relatively cheap (there’s an “unlimited” option for $45 per month), but you’ll have to deal with any issues that come from having your phone constantly go between Verizon’s network and Comcast’s hot spots.

Disclosure: Comcast is an investor in Vox Media, The Verge’s parent company.