None of the new iPhones will be able to go online using a spectrum band known as AWS-3, according to Apple’s specifications page. Carriers spent almost $45 billion last year at a federal auction for the rights to use the AWS-3 band, also known as LTE band 66, and are starting to put it in service. The carriers all also operate on other high-speed bands that will be iPhone 7 compatible, but they won’t be able to connect to the new phone with the additional AWS spectrum.

For consumers, that means their new iPhones may not be able to jump onto a band that is apt to be less crowded, and possibly faster. And Apple also made a choice that limits the ability of some new iPhone 7 models to connect to all four major carriers.

That’s because Apple split the manufacturing of iPhone modem chips between suppliers Intel and Qualcomm. Modems from the two companies aren’t interchangeable, effectively eliminating the ability of some iPhone 7 models to connect to all four major carrier networks. Although the two modem chips have many of the same features, Intel’s chip can’t connect to older CDMA wireless networks still in use by Verizon and Sprint for voice calling.

Qualcomm (QCOM) and Intel (INTC) declined to comment.

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Despite the complications, Apple (AAPL) likely had sound reasons for making the trade-offs. The company didn’t respond to several requests for comment, but adding Intel as a modem supplier was an effort to become less dependent on sole supplier Qualcomm, which owns many patents related to CDMA. And adding modems with additional spectrum bands like AWS-3 could be more expensive, use up more power or require other unwelcome trade offs.

Sprint (S) customers, at least, won’t be affected by the lack of AWS compatibility. The carrier didn’t participate in last year’s auction, so it doesn’t own any of that band. “We are extremely excited about the iPhone 7 because is allows us to leverage all three of our spectrum bands,” John Saw, the carrier’s chief technology officer, told Fortune.

Perhaps not surprisingly, AT&T (T), Verizon, and T-Mobile were less forthcoming about how the iPhone 7 would work on their networks, pointing to Apple’s web page listing the new phone’s specifications instead of granting interviews or providing detailed information.

The problem caused by the split between Intel and Qualcomm can be avoided by savvy phone shoppers. Customers who want to be sure that their iPhone 7 will work on any of the big four networks, preserving their ability to switch carriers, should opt for a phone with a Qualcomm chip, which encompasses the models being sold on the box as Verizon @verizon (VZ) or Sprint compatible. They are labeled as models A1660 for the 4.7-inch iPhone 7 and A1661 for the 5.5-inch iPhone 7 Plus.

For more on the introduction of the iPhone 7, watch:

All of the new iPhone 7 models are said to be compatible with carrier aggregation, the most popular technique that carriers are currently using to improve speed and efficiency on their LTE networks. The technique allows a single phone to tap into two or three different spectrum channels at once, allowing faster downloads.

However, it remains unclear whether the iPhone 7 line can use some of the even newer techniques, known as 4X4 MIMO and 256 QAM, that carriers will turn to next to speed up their networks. T-Mobile (TMUS) has said it was adding those features, but declined to comment on whether the iPhone 7 could take advantage. It has said that the latest Samsung phones, the Galaxy S7 and Galaxy S7 Edge, are capable of using the faster speeds.

T-Mobile chief technology officer Neville Ray told some customers on Twitter that the carrier hasn’t heard from Apple yet on the compatibility issue. T-Mobile is “waiting for them to confirm” whether the new phones will support carrier aggregation and MIMO, he said.