How can I find if a phone is locked or unlocked?

Nice catch, dst11. I've never used TruPhone, but you might be correct about both not being a true AT&T MVNO, and requiring an unlocked phone. (I missed that entirely.) I think we have some TruPhone users here, and maybe they can confirm.

Exactly so for "Any other branded device":
And yes, I'm afraid I have more than 2 obsolete phones.... Significantly so.... Would you like some?

I'm afraid I'll have to pass. I have more than enough obsolete computer stuff laying around!

Anyway, I completed page 1 of the unlock request, completed page 2 (I left the 'mobile number' field blank/empty since I didn't have any such info) and submitted it. I clicked the confirm link in the email I got. The line below it says "Want to stay updated? Check the status of your request" and I clicked that too. Here's the text of the response:

Does that mean the phone was bought less than 6 months and then the buyer listed it on eBay where I bought it?

Yes, it means the phone wasn't active on an AT&T plan for the required 6 months to be eligible for unlocking. I don't know if calling the AT&T number suggested by SuccessOne would get an agent to inform you how many months paid service are still required.

I apologize for leading you down the wrong path by using the other-carrier SIM method. This has worked reliably for me in the past, but I've only done it on Android devices. Maybe KaiOS doesn't notify as aggressively about rejecting a SIM when still locked?

Yeah AT&T is very strict about not unlocking prepaid phones that haven't met the 6-month service requirement. At least you are able to know right away through their unlock portal. I wish T-Mobile had something equivalent.

I'll try that!

Please don't worry about it at all. Who cares if the phone is locked? I got it on eBay for $15.5 and I ordered a truPhone SIM so I can put it on my XP5, and transfer the FreeUp SIM from the XP5 to the Smartflip.

The buy-it-now price on the Smartflip was $45, but there was an auction too starting at $15. I put a $15.5 bid and ended up winning it.

I called AT&T and 'customer service' asked for the IMEI and told me 6 months. I couldn't understand the rep clearly. Anyway, I'll put my FreeUp SIM in tomorrow and we'll see how things go!

Success! Exactly like KentE described. I took out the FreeUp SIM from the Sonim XP5, removed the Micro SIM "ring" to turn it to a Nano SIM, and inserted that to the Alcatel Smartflip. 5 seconds or so after I opened the phone, it connected with the cell tower and it works fine. I tried calling and texting and both were fine. Now I'm waiting for the truPhone SIM to be delivered to put in the XP5.

BTW, when AT&T said "We can't unlock this device because it hasn't been active for at least six months of paid service. You can submit a new request after six months of paid service." does time on FreeUp count or it must on AT&T itself?

Unfortunately, time spent on an MVNO never counts towards carrier unlocking. Only months directly on the branded carrier will count.

I used prepaidcompare.net and the cheapest ATT plan is $8.33 a month. Is that the cheapest available ATT plan? If so, since you can use the phone, even when it's locked, on most ATT MVNOs, I think the Smartflip will remain locked!

So, what happened was the eBay seller got the Smartflip cheaply from ATT, because the phone was bundled with monthly service, but then the seller cancelled the monthly service and listed the phone on eBay? Does that sound OK/plausible?

If you could get on the Gophone plan that charges $2/day only on days it's used, make sure voicemail was disabled, and then ignore it, then the plan would technically be cheaper, but I think in order to keep your prepaid credit from expiring you'd have to pay at least $50 ( $25 for 90 days without expiring twice), so even if that didn't leave you short by at least a day, I think it wouldn't effectively save you anything.

I think the cheapest AT&T plan is via Tracfone and their Ebay store.
This annual pre-loaded SIM card (1200 minutes, 1200 texts, 3GB data) for $50/year. Tracfone Prepaid Wireless Smartphone Plan+SIM-1200 Min,1200 Txt, 3GB Data 616960284093 | eBay
At checkout, apply coupon INSTANT20 to reduce the cost to $40 (plus tax). Works out to around $3.50/mo including tax.

If you sign up for the Tracfone Rewards Program with an existing user's referral code, you can get up to 8000 bonus points that can be 'spent' according to your needs: you could add an additional year of service (with slightly smaller buckets), or just extra minutes (up to 4000 minutes), texts, or data. There are a number of forum members with Tracfone referral codes in the referral thread: https://www.nthcircle.com/forum/general/76-referral-links-allowed-in-this-thread-only-hungryghost?start=100

dst11, I think I remember that phone sold very cheaply a couple of places on a Black Friday deal. I'm not sure that AT&T Prepaid usually does bundled phone offers, but maybe so.