Buying a smartphone from a third party site like Swappa or Amazon can save you quite a few dollars, especially if you buy it used or refurbished. Also, big box retailers like Best Buy sometimes offer deep discounts on new handsets you may not be able to score by going direct through the carrier. But keep in mind, not all smartphones you buy from third parties will work on just any carrier.
This brings up the issue of compatibility, and the discussion of the differences between GSM and CDMA. T-Mobile and AT&T utilize GSM technology for their handsets, while Verizon and Sprint use CDMA. The practical difference for users used to boil down to the fact that GSM smartphones came with removable SIM cards that could be swapped out to work with other services, whereas CDMA smartphones typically had no SIM cards, with user data stored with the carrier.
This severely limited smartphone and carrier compatibility. AT&T and T-Mobile users had the most success in switching to one another, but most users were stuck with their service unless they wanted to pony up for a new smartphone. Carriers also locked smartphones to their respective networks, especially if they were purchased under with a subsidy under contract, making the process of switching all the more difficult.
The good news is that things are loosening. Thanks to the FCC, all of the major carriers are required to enable smartphone unlocking so that it can be taken to another carrier. There are still some limitations that require you to meet the terms and conditions of any service contract you sign, but by law the carriers are now required to assist you with unlocking your smartphone.
Buying an Unlocked Smartphone
In your search for the perfect smartphone, you will come across numerous third party sites and manufacturers that sell their phones already unlocked. An unlocked smartphone is one that doesn’t have to be used with a specific carrier. In that sense, you’re not “locked in” to a particular service and you have the freedom to choose and switch to another if you’re not happy.
Things get a bit complex when it comes to unlocked smartphones, however, because of the aforementioned GSM vs. CDMA incompatibilities. It was once the case that an unlocked GSM device was limited to carriers that work with the device’s technology. The vast majority of unlocked smartphones were GSM. For this reason, buying an unlocked smartphone meant you could move between GSM-based carriers, but you still weren’t be able to use it with CDMA carriers like Verizon and Sprint.
There were and still are exceptions to the rule. The iPhone 6 and 6s, Motorola’s Moto X Pure Edition, and some Google Nexus smartphones are recent devices that support both GSM and CDMA. In fact, many devices were and are manufactured with both CDMA and GSM antennas and one not active depending on the carrier. It’s simply cheaper this way, rather than make unique devices for the various networks.
Regardless of GSM and CDMA origins, most new phones manufactured today include SIM cards, and here’s where things get muddled.
In the case of Verizon and Sprint, the carriers still use the CDMA method of verifying their subscriber service against an internal whitelist of accepted devices. The reason for the SIM card is LTE. The current networking standard, often referred “4G” as it is widely considered the fourth generation of consumer cellular networking technology, requires a SIM card in order to operate.
This growing convergence to 4G LTE support means that some GSM phones could work on CDMA networks, in theory, provided the phones support the proper LTE band.
LTE networks are grouped by frequency bands. Each carrier shares slices of the proverbial LTE network pie, three to four bands each, with some overlap occurring. Each has a main band that handles most traffic, with the others dealing with overflow, speed and signal boosts, and/or stability.
Bands 2, 4, 5, 17
Frequencies 1900, 1700 abcde, 700 bc
Bands 25, 26, 41
Frequencies 1900 g, 850, 2500
Bands 2, 4, 12
Frequencies 1900, 1700 def, 700 a
Bands 2, 4, 13
Frequencies 1900, 1700 f, 700 c
If you want to buy an unlocked smartphone, one of the first things to do is determine which carrier will support the smartphone of your choice. This is accomplished by looking at the smartphone’s spec sheet, then comparing the supported LTE bands with those of the carriers. You’ll want to match as many bands as possible, especially the main bands. For AT&T, that’s band 17; Sprint, band 25; T-Mobile, band 4 and 12; Verizon, band 13.
And there’s more. Remember that whitelist we mentioned? This particularly true with Verizon, if it hasn’t whitelisted a phone, it can muck up things for users looking to activate that device, regardless of support. This happened with some Moto X Pure Edition and Nexus units just after launch. There’s a workaround that involves activating an LTE SIM card on another smartphone that has been whitelisted, and then inserting it into the non-whitelisted smartphone.
Wait, there may be even more. Though most smartphones already have this information in the system files, you must sometimes dive in manually set the Access Point Name (APN) settings. The APN is basically the gateway between the cellular network and any other network, like the internet or a VPN. The carriers use the APN to set things like IP addresses and security.
While this seems like a highly technical and involved process, it’s not. The carriers are happy to provide the proper settings, and users can enter them easily enough via keyboard by diving into the device settings, typically under “wireless” or “networks,” and then “mobile network.”
Voice Over LTE
Even if you manage to connect a CDMA smartphone on a GSM network, or vice versa, you might not get voice call or text support. This is because most carriers still use the old standard for these signals. In fact, many use the old second generation GSM and CDMA networks.
Thankfully this is changing, as carriers look to move on from GSM and CDMA. In addition to Wi-Fi voice calling, FaceTime, and Skype, as well as messaging apps like Google Hangouts, iMessages, Facebook Messenger, and What’s App, carriers are transitioning voice data to the LTE network.
VoLTE, short for Voice Over LTE, also sometimes branded as HD Voice, provides users with clearer calls that connect faster. Voice latency is reduced, as is the common crackle and hiss. That means VoLTE calls sound better, and callers step on each other less during conversations. It also better serves smartphone batteries, as smartphone won’t need to switch antennas from LTE to GSM or CDMA. It will also result in simpler, data-only smartphone plans.
Prior to VoLTE, some carriers marketed an “HD Voice” feature that wasn’t VoLTE-based. Rather, it was just simply a tech that improved voice call quality.
For futureproofing, make sure any new smartphone you buy supports VoLTE. Carriers are rolling it out region by region as of this writing.
SIM Card Sizes
Not all SIM cards are made equal. This can be the source of additional headaches if you want to swap your SIM card into another handset, or drop a prepaid SIM card into your phone for use overseas.
SIM cards come in three different sizes: standard SIM, micro SIM, and nano SIM. Most new phones are nano, but some mid-range and budget handsets still support micro.
More recently, a fourth size, eSIM, has been spotted in the wild, appearing in the Samsung Gear S2 smartwatch. eSIM cards are essentially embedded cards that are programmable for use with different carriers. The benefit of eSIM will be that you won’t have to physically swap out your SIM card if you want to use a different carrier service. This also saves on precious space as manufacturers looks to make lighter and thinner devices.
You can contact your carrier and ask for a SIM replacement if you buy a handset requiring a different size SIM, but this may incur additional expense. You can actually cut a micro SIM into nano SIM size and have it work in a smartphone, but that sort of DIY tinkering is best left to advanced users. You can also find SIM card adapters on the market, but keep in mind that some smartphones are incompatible for use with adapters. These are generally smartphones that utilize the push-in card slot mechanism to insert and remove SIM cards.
Want to learn more about buying a smartphone? Read all about network technology, including LTE, GSM, and CDMA. And then read all about no-contract smartphone data plans and pricing. Finally, learn all about smartphone specs, the differences between iOS and Android, recycling your smartphone, and choosing the right data plan.