All these reviews are hilarious.
5-stars: like someone said, "what's up with all the 5-star reviews?" I agree.
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2-star and 1-star: I these people don't know how cell phones and cell networks work.
Here's my experience. I'm very pro-privacy so I know a thing or two about how the phones work, how the operating systems work, and how the networks work.
I have 4 Tello cards. Long story short, I was only able to activate the first card (as a test). My intentions for this service is to have cellular backup, using cellular routers instead of cellular phones for when we have ISP outages, which we have experienced more frequently in the last couple years: slow is better than none.
I had no issues with the first card. The card costed me $2 to my door, then a $10 plan for 2GB and a phone number, for testing. One of my kids is currently using it as a backup phone, so it is currently working. About the phone, it is an eBay special Motorola G7, blah blah blah, unlocked, I flashed LineageOS 21 on it. It's working fine.
The next card I tried activating without a phone number, 2GB data-only. This card showed "Pending" for 24 hours, then dropped off my history. Keep in mind, there is no phone involved at this point. The third card, same issue. I even set up a new account to activate, since Amazon will only sell you one card (per purchase), so I thought maybe Tello only allows one card per household. Long story short, I was only able to activate the original card. The other cards are destined for the rubbish bin.
Just some education for those who aren't aware, your old AT&T phone, for example, won't work with Tello unless you unlock it first. They are "carrier-locked", to bind you to the service until the end of your contract. Your old T-Mobile phone will probably work, since Tello works over the T-Mobile network, I've never used T-Mobile so I couldn't say 100%.
As far as frequencies, I bought a Xiaomi phone about a year ago, hoping to use a really nice phone, but apparently, the frequencies they use don't match US carriers, so some phones like the Xiaomi phones, none of them will work at all, due to the radios. Yes, check to make sure your phone uses US frequencies. If your bought them in the US, they will probably work.
I have a Brax Mototola G7 phone and they reset the IMEI, so as far as I know, all the carriers say the "IMEI is invalid", otherwise, your IMEI should be fine is you haven't messed with it. MOST people are not savvy enough to mess with the IMEI, so "invalid IMEI" is pretty rare.
As far as customer service, the activation shows "Pending" for about 24 hours, then drops off my history. The last card I tired said "Pending", then "Refunded". You have to option to call them or email them. When I reached out to them via email, they asked to verify my identity. At face value, this seems reasonable, but having not been able to activate 3 out of 4 cards, already having my name, phone number, address from my first card, what more personal information do they want? Let alone, "they don't know me", what more information can they use to "verify me"? Personal data harvesting much? Back to the "Refunded" comment: their website says you don't get charged until you are successfully activated. The last card I tried was never activated, why would I get a refund, unless they charged me before a successful activation?
In conclusion, if you are lucky enough to get one of these cards activated, it is fine as long as you're not doing anything demanding with it. I don't use SMS for privacy-reasons either, so I would only order data-only for these services. Perhaps they don't want to sell you "data-only plans". I'm off to find another service and throw all these cards away.