What happens to sent SMS's that are undelivered b/c phone is off / out of country?

Question

I'm traveling in Europe for several weeks, and my AT&T GoPhone doesn't support international voice or data roaming outside of US & Canada.

I want to know what happens if someone sends me an SMS message while my phone is off / out of the country.

- Will it get returned to them as "undeliverable"?

- Will I get it delivered to my phone once I get back to the US?

I've gotten very mixed messages about this.

Some reps have said "YES, when you turn your phone back on in the US, every message that was sent to you while you were away will get delivered."
Some reps have said "NO, you won't receive any SMS that was sent to you while you were out of the country." This answer, btw, doesn't make sense to me, because when I turn my phone off, or it runs out of battery (and is therefore OFF), when I turn it back on, I *do* receive any SMS sent to me while it was OFF, right? So why wouldn't I receive them simply b/c I was out of the country.
Some reps have alluded to a "time limit" that my phone must "receive" the SMS within, or else it will be lost forever. I haven't been able to get a firm answer on just what the time limit is, but the general principle is that if I turn my phone on within, say, 48 hours of the SMS having been sent to me, then I'll receive it...but if it's over 48 hours (if 48 hours is indeed the time limit) then it'll simply be lost forever.

That 3rd explanation at least makes sense to me...though I have no idea who is telling me the truth and who has no idea what they're talking about.

Help?

GeekBoy · Answer

The real answer is there is no way to tell.

SMS messages are not guaranteed to be delivered even if you are on the home network and you phone is turned on. They are designed to use "spare bandwidth" to deliver them, so they may get delayed if they pass through an "over utilized" segment of the network.

Most likely, they will queue up and all be delivered to you when you get home, but there are some chances that they may just vanish into the net work never to be seen again. Now, if you have an Apple device and are using iMessage, they are more likely to be queued and delivered later.

pleomorph · Answer

In general, AT&T will keep the message on their servers for 72 hours and will keep trying to send it to your phone. After 72 hours, the system "gives up" and the message is deleted. The sender likely will not receive any sort of notification that the message did not go through.

I found the following information in the AT&T developer documents:

"The AT&T Wireless network retries deliveries of SMS messages to the AT&T Wireless customer for up to 72 hours; after which time any undeliverable messages are deleted from the AT&T Wireless network."

Source: https://developer.att.com/apis/sms/docs