[Android Q&A, Help & Troubleshooting] jb_wisemo:
In most of the world, the "GSM-style" standards are used. The phones are completely the same, the only thing that is truly carrier specific is the "SIM card", which contains your phone number and a code that allows the carrier to bill the calls to you. However phones can be "SIM-locked", meaning that for the duration of your contract, the phone will refuse to accept a SIM card from a different carrier, unless you pay the carrier to get out of the contract, in which case they must tell you the unlock code for your individual phone.

There are however approximately 3 parts of the world that use different radio standards, and those are sometimes made with slightly different chips or circuit boards:

- The US/Canada carriers that used to be based on the old CDMA signal standard. For a very long time, those kept making their own slightly different signal standards, and sometimes based billing on the phone serial number, not a replaceable SIM card. Note that some other US/Canada carriers use the international "GSM-style" standards.

- In Japan, there were, and maybe still are, different signal standards such as "i-Mode".

- In Mainland China, they might also still have their own standards.

So some manufacturers might make up to 4 different variants of a phone, to avoid the cost of including the parts for a region where it won't be sold. But the only thing that is customized for individual carriers is adding extra software and maybe deliberately refusing to talk to competing networks. In theory, they could also fine-tune some bug workarounds for oddities in their radio network, though I personally suspect this is mostly just an excuse to make the other changes.

Note that electrically, while the core chips might be able to do any standard with just different software, sometimes the following is done with region-specific manufacturing:

- Fine tuning tiny antennas, coils and other old school radio components to boost the frequencies used in a region.

- Extra circuits inside the chips to do some of the region-specific signal decoding/encoding in hardware and not software, I suspect this was more common 10 years ago than now.

- Adding receivers for Russian (GLONASS), Chinese (ATLAS) or European (GALILEO) competitors to the US Military GPS satellite signals.

- Printing the sticker with the serial number etc. in a language other than English.

- Paying competing phone makers for permission to use their patented phone technology. A manufacturer might save a few bucks per phone if they don't have to pay for some invention used only in the US or Japan, even if almost all the parts are still there. This of cause does not apply if they managed to negotiate a good deal. Remember that Google payed *billions* of dollars to get the Motorola phone patents (they also got some phone factories in the deal, but they sold those later), and Microsoft made similar deals with Northern Telecom and Nokia.

This is also why people from outside the US/Canada sometimes don't understand what North Americans are talking about when they mention the "AT&T Stone" or the "T-Mobile G2x" phones, because in the rest of the wold, you buy a phone branded by a phone maker such as Motorola Razr or Samsung Galaxy 6 Edge or Apple iPhone 6S, and then look around for the best contract to get that phone cheaply. Kind of like what US/Canada people do with Apple iPhones. In some places, you actually go into a phone superstore, pick a phone and then read a price tag saying "This much without contract, this much with T-Mobile contract, this much with Orange contract", then go to the counter and sign the chosen contract while the store clerk types in a code to lock your new phone to your new contract.

And finally of cause, this is the reason why the "radio" part of the phone is not part of the amateur ROM kitchen: Because the chips can send out almost any radio signal with the right software, only government-licensed manufacturers are allowed to change that code, because the FCC (and its overseas counterparts) doesn't want a bad ROM to start jamming police radios or even just block the phone network by refusing to stop when it is another phones turn to send a few bits.