Smartphone makers and carriers have been making big promises about next-generation 5G networks that are supposed to make our smartphones blazingly fast. Motorola on Thursday claimed the prize for announcing the first 5G-ready smartphone. But there are a few caveats.

For one, the phone — called the Moto Z3 — is actually a 4G phone, coming Aug. 16 on Verizon. Its 5G capabilities will come via an accessory that Motorola plans to release sometime early next year, which will snap onto the phone’s back.

The Z3 will cost $480, with an option to pay $20 for 24 months, Verizon said. Motorola and Verizon did not announce how much a 5G data plan or the 5G accessory will cost.

The add-on is Motorola’s 18th “Moto Mod” accessory, which people can buy to add some sort of hardware boost to the company’s relatively inexpensive smartphones. Other optional accessories include a projector that can be attached to a phone to beam its display onto a wall, more powerful cameras and additional battery packs. Existing Moto Mods cost anywhere from $50 to $200.

The newest mod could actually extend the useful life of the Z3, thanks to the new network compatibility. All the major U.S. carriers have announced plans to build the faster networks, which require phones to have different antennas to take advantage of those benefits. Adding the 5G capability with an accessory should make the transition cheaper, as it negates the need to buy a new phone.

Motorola promises the 5G-ready Z3 can download 90 Spotify songs in 10 seconds. The accessory will also include a battery pack to ensure running on the new networks doesn’t leave you without power.

Of course, the value of a 5G upgrade depends greatly on whether your city has access to a 5G network. There aren’t very many cities in the United States that are scheduled to have 5G coverage from Verizon, which said in Thursday’s release that it will bring the new technology later this year to “Houston, Los Angeles, Sacramento and one additional city, to be named.”

Motorola’s mods have seen reasonable adoption among the company’s customers. Motorola officials said in a press briefing that about 40 percent of Motorola smartphone customers use a mod. Of the people who purchase one, 60 percent buy a second or third.

In the United States, Motorola is the fifth-most-popular smartphone maker, behind Apple, Samsung, LG and ZTE, according to Counterpoint Research.

The company said it is also working to make the 5G Moto Mod compatible with older Motorola Z-series phones but is focused solely on the Z3 for the time being.