All four in my family take their GSM cell phones (T-mobile) to Thailand and simply switch out SIM cards. No problems and both US and Thai contacts are kept. Plenty of unlocked phones for sale on Craig's List from resellers. The carriers are somewhat loath to sell unlocked phones but it is very easy and cheap to find people in Thailand to unlock your phone if you don't want to bother doing it yourself. Cell phone theft is rampant in Thailand so yes, not a bad idea to buy a cheap phone in Thailand and use that phone when going out at night or visiting high risk tourist sites as that newer model iPhone or Samsung will attract attention.

I bought my wife a Samsung S6 phone prior to travel to Thailand last fall. It was an AT&T version (GSM), but purchased without the contract. In order to use the phone in Thailand, you only had to activate a service in the USA (for us, Metro PCS) and use the phone for 5 to 10 minutes. This unlocked the phone for international use. Then upon arrival in Bangkok, just went to the True booth at the airport, and the saleswoman removed the Sim from America, gave that to my wife in a small case, installed a True Sim, and after paying $B400, the phone worked like a gem. That $B400 lasted my wife the entire 7 week trip, including all data usage. When she return to the USA, I removed the True Sim and replaced it with her original Sim, and the phone worked fine. Very simple and trouble free.

The U.S. still has CDMA? How quaint.

I'm probably the only reader that does not have a cell phone. To keep from looking out of date I hang my old land line dial phone around my neck whenever I go out. Never get disturbed by any calls while I'm enjoying a cold one.

Again, not always. Not all quad band world phones seem to work here on AIS or DTAC. No problem with True yet though.

Never had the slightest problem with either DTAC or AIS when using phone or 3G sticks coming from Europe.

Well, actually I did sometimes with DTAC, because they still use 850 Mhz in rural and semi-rural areas, which some of my equipment didn't support. Note that True does, too.

On the other hand AIS only uses 900 and 2100 Mhz.

I'm only speaking of 3G here. I don't use 4G yet. Maybe that's what you were referring to?

Considering I said "some" phones, not "all" phones.......

And no, it has nothing to do with 3g or 4g. AIS and DTAC Sim cards simply don't work with many quad band world phones for whatever reason.

Will a US cell phone convert to a Thai phone?

I don't know what to think. Only an American can formulate a question this way.

why bother, phones in Thailand are dirt cheep and come with minutes

Maybe your crappy phone, but the OP probably has something that doesn't suck.

Most decent phones these days are quad band LTE and work on both CDMA and GSM networks. If it's unlocked then it will work - otherwise it will have to be rooted, then it will work. So yes.

Even if it is unlocked and has LTE band there is a likelihood some features will be locked out.

I expect it would work as a phone and access data too on 3G.

And example is Verizon block tethering and using the phone as a Wi-Fi hotspot, and may have removed the Radio App.

You need to find out the exact model number of your phone in options and google what frequency bands it supports, "quad band" means jack shit when theres so many different bands these days

Im with true and they use 850, 2100mhz for 3g and 900, 1800, 2100 for 4g. Band 1 (2100mhz) is the most used in thailand so if it only supports that and you dont take the phone out in the sticks then it will be enough

This will tell you all you need to know, enter your phone, and then country and it will list the bands used for each carrier

https://www.frequencycheck.com/models