Possible Carrier Locking of Devices Using "Carrier Optimization"

A long time ago (From the mid 90's till about 6 years ago, as of the writing of this review) carriers would sell devices/phones that were locked to the parent carrier.  (eg Verizon Phones were locked to only work with Verizon, same for at&t, Sprint, and T-Mobile.)

Shortly after the rise of smart phones and changes to services to be more data centric, the FCC passed a law to prevent carriers from locking devices AND not allowing their customers to unlock those said devices.

How the law was written:
Carriers are allowed to sell devices locked into their own networks
Carriers must allow their customers to unlock their phones within a short time frame of the phone being activated.
However, the law also allows a carrier to "Optimize" the devices they sell for usage on their own networks.

So what is Carrier Optimization?
According to the carriers, who have all admitted openly to at least consider Optimizing the devices they sell;  The devices sold at carrier stores and their licensed partners are optimized to work best on the carrier that it has been optimized to work with.

Essentially, the phone/device may be the same exact model number as sold on other carriers, but uses a different base band version and Software Channel that is specific to the carrier in question.  (For instance it comes bundled with software specific to the carrier, like Metro's "MyMetro" app or similar apps put on by Verizon, T-Mobile, at&t, ect.  This software may also be pre-installed as system software, so it cannot be removed.)  However, most optimizations tend to also remove any technologies, frequencies, or signal bands NOT used by the carrier who provided the device.

We have seen evidence of carriers using this optimization to ensure that the phones will only work best on their own networks only.  By only including the signal bands used by their own network, this ensures that the parent carrier can "lock" the device, and their customers, onto their own network.

Some Bands may have some overlap, but are still largely incompatible between carriers.

We have also noticed that LTE on GSM is not the same as LTE on CDMA. LTE still is carried by one of these two main base technologies, and appears to be an upgraded band that works on top of LTE and GSM. My newest device is a Tello phone, which uses Sprint's network, and have installed a Cellular Signal tracker to confirm this information. My signal shows LTE, but the app shows "LTE (CDMA)" which indicates that the LTE signal is being carried by the CDMA towers.

This means going from a CDMA carrier to a GSM carrier is still nearly impossible. AKA switching from Sprint/Verizon to at&t/T-Mobile and vice versa cannot guarantee that your current phone will work on the new carrier well, if at all. (However, T-Mobile does seem to operate some CDMA, perhaps from the MetroPCS network.)

This means that the phone will work only with bands that are overlapping between carriers and can purposely degrade the coverage of the device if it is unlocked and used with a competing carrier.

Our evidence of this happened just 2 years ago.

I personally own a phone bought from Metro by T-Mobile (Formerly MetroPCS) and have not had any issues with its coverage or signal anywhere I use it.
Mind you, Metro is the Value Brand of T-Mobile and is supposed to be on a lower priority of that of actual T-Mobile Customers.  (Which has never affected my ability to use this device or degraded my signal any as of yet.)

I had a friend who had an iPhone 7 at the time that he bought from Verizon.
Verizon and T-Mobile do have some network bands that overlap in our area, but T-Mobile and Verizon also have Network bands that only they use respectively - therefore there is no overlap.  Also this means that his iPhone may have been Optimized by Verizon to only work with Verizon's Network and may work with any technology that Verizon AND other carriers use at the same time.  In this case, that means that signal bands that are used by both Verizon and T-Mobile can be used by his iPhone.  However, the bands used by T-Mobile that isn't used by Verizon were removed from that same phone, even though Phonescoop lists those bands as available - leading T-Mobile to believe that his iPhone should work on their network as well.

We found out that his Verizon iPhone did not properly work on T-Mobile's network after he unlocked it and put in a T-Mobile SIM.  Possibly because the phone was Optimized ONLY for Verizon's Network.

Essentially, his iPhone would show 0-1 bars of service in the same exact location that my Metro (and my husband's actual T-Mobile) phones would have 5-6 bars of service.  Data on his phone would fail to work in areas where we were browsing just fine.

So through this example, his phone's coverage was greatly diminished by not actually having the full list of bands it should have had, simply because his phone was Optimized for Verizon.  Just like our phones are Optimized for T-Mobile (Since Metro is owned by T-Mobile).  So we may have a similar experience if we were to try taking our phones to Verizon or at&t - that our phones will simply fail to work as well as they do on T-Mobile.

What this means is that Carrier Optimization can be used as a means to lock devices to the carriers that sell them, even if you - as a customer - are legally allowed to unlock the device to use with other carriers.  By merely making sure the device doesn't work as well as it should on competing networks.  This can make customers think that said competing network doesn't have as good coverage as they should (or at least claim to) have.  And with our own knowledge that almost all carriers only have a 1-5% difference in their coverage on the national scale, you should be covered almost everywhere by nearly all of the National Carriers.  (Keyword is "should"  - we are aware there are smaller areas where one carrier does offer better service than the others.

This means that Carrier Optimization allows a carrier to make its customers think the competition isn't as good as them and therefore allows the aforementioned carrier the ability to degrade the phones usefulness on competing networks.  This pretty much guarantees that the customer will return, since their now favorite phone doesn't work on the competition.

So is Carrier Optimization legal?
Currently, yes.  Since the only thing admitted by carriers is that Optimization allows them to ensure that a device they sell works best on their network.  Since each carrier has contracts from the phone manufacturers where they can have the maker Optimize the phones they sell.  However, this does allow the carrier to have the maker remove advertised technologies/bands from devices not used on the carriers network - and optimize the device to work best on that same network, and only that same network.  (This latter part is not openly admitted by any carrier, but is highly possible.  If it ever did come to light that this is actually happening, Optimization may also become illegal as it stands.)

How can I truly get the best from all carriers?
You can only be able to carrier hop and use all of their networks is with a Truly Unlocked phone.  These are usually sold only and usually also sold directly from phone makers without all the carrier add-ons installed.  If you buy from your carrier, your device may be "locked" to only work well on their network.
If you buy second hand, it may also be Carrier Optimized.

Usually you can tell if a phone is Carrier Optimized by the software is comes with, the base band version, and/or the software channels listed on the Settings > About Device pages on your phone.

While we know carriers can Optimize the devices they sell to work best on their networks.  This may allow them to remove any tech/frequencies/bands they do not use - which could be things used by competing carriers, which would make the phone not work as well on competing networks.  We only currently have the one example to show this may be the case, but is not proof that it is truly what is happening.  But taking into account at least one carrier did actually sue the federal government to block the passage of the law requiring them to allow customers to activate phones, then it is possible they may be using this as a backdoor technique to "lock" a phone onto their own network - while still following the law by allowing the device to actually be unlocked.