Suppose a person purchases all the chips and components and has the know-how to solder up a homemade roll-your-own working mobile phone.
Would the major mobile service carries allow you to purchase their service and connect your device to their network? I’m guessing no, they cannot trust it to play nice with their network or not be purposely malicious.
I suppose for every model phone they support there is a certification process, and they are not going to bother to allow any shmuck with chunk of plastic to go through it. Or would they, perhaps for a price?
I suppose you know you can switch sims from one device to another. Including switching those sims from the grandfathered unlimited plans to wifi devices so one can run their home off of one plan. Even buying off of ebay a used Verizon WiFi hotspot and slipping my grandfathered unlimited plan sim into it and using their own device and their own service in a way they didn’t like. Given that and Android is free to use and modify, I’m not really seeing a way they could do that.
However I don’t think you could walk into their store with such a device, but have to start it on another device than switch the sim card. And that may be the weak spot as more and more is going to e-sim, but if you walk in wiht a device that requires a sim card I think you are good.
A spokesman for Cingular Wireless stressed that any cell phone radio has to be approved by the Federal Communications Commission and the carrier itself before using the network
This feels like a hard stop to the spirit of the opening hypothetical. A tinkerer isn’t going to have the connections, resources, money, or time to run the bureaucratic certification obstacle course, and it would be a violation of law to put it on the network without that (even if SIM-swapping reindeer games would work).
And we don’t discuss violating law here. An implicit but unshakeable condition of OP has to be “legally”.
If you can convince their network that the phone is valid, they won’t be able to stop you. But that is a big if. The phone itself carries with it its own unique identity The International Mobile Equipment Identity. That codes manufacturer, model, and country of origin as well as its unique id in that space. No IMEI, and you won’t be connecting to a network. Exchange of IMEI forms part of the basic cell protocol. Now you could fake or clone one, but you will almost certainly find yourself in breach or a whole raft of contract clauses and local regulations, and possibly laws. One assumes the OP’s question is about a legitimate effort.
You can be pretty sure that the rule, if you asked, is going to be no valid IEMI, no connect. If you can get a valid IEMI, you are in a sense already passing the barrier of guaranteeing that the phone plays well.
I’m not an expert, just what I’ve heard over the years:
First, yes, on certain radio bands I believe there is an FCC certification process - not just that it works properly, but that it does not emit stray radiation that messes up other people’s equipment (was more important in the days when broadcast radio and TV were most used). I assume the 10mW exception for unlicensed devices like walkie-talkies may apply, but even those were limited to certain bands. Same with amateur radio equipment - roll your own, but only for certain bands. The goal is that everything plays nice with others.
Second, cellphones have not only a SIM card with “serial number” but the phone itself has an IMEI “serial number” tied to the hardware. Not sure how those are allocated, but spoofing someone else’s (“cloning”) is, I believe, a crime. Not sure how you’d get your own, or whether using one from your own defunct phone is legal (I’m guessing not).
Finally, not just the chips but the programming. There’s specific handshake, and today, crypto. We could learn about Prince/King Charles’ deep desire to be a tampon years ago because original analog cell broadcasts were in the open, with just a data burst at the start to identify the phone. Modern systems are encrypted, multiple phones will share a channel inserting their data packet in with others; the packets are encrypted so the average Joe can’t intercept the royal booty calls any more. Presumably you need access to certain encryption keys. Then it needs to determine which bands the radio uses… 3G, 4G, 5G or other protocols.
Whether buying the “right chips” will include all that necessary programming, I don’t know, But this gives you an idea of the challenges.
There are a few different levels of assembly that are going to matter.
It is easy to buy an m.2 (and other) form factor 5g radio card. This radio card could then be put in a Raspberry Pi, laptop, desktop, etc. to create a phone. In this case the radio card is the part that was assigned an IMEI number and received FCC certification. With the appropriate SIM card the 5g radio will connect to a mobile provider. You’ll then need the appropriate software on your laptop, Pi, etc. to use the mobile network.
You could possibly go one step lower, though. The 5g radio card itself is made of components. Buying those components is probably not too difficulty, but (and I don’t know) it may then be on you to get an IMEI assigned and seek FCC certification for your assembled radio device.
So my suggestion if you want to build your own phone, is to design your system to use a commercially available 5g radio card.
For example, the Pinephone people have built a phone by essentially putting together off the shelf components.
Like others have said, there’s the challenge of getting FCC approval.
A homebrew cellphone is a little radio station. There’s only one radio spectrum, and everybody has to share it. The intended emissions have to cooperate properly with the other users, and the emissions at wrong frequencies have to be small enough not to interfere with users of those frequencies who aren’t wrong.
If we all shared a party line, you’d want it to be hard for other people to start inventing stuff and connecting it to that line.