Apple’s iMessages work fine in normal circumstances, when everyone has data coverage. They are delivered promptly, and avoid charges for sending or receiving SMS text messages. They can create a communications black hole, however, when the intended recipient doesn’t have data coverage, as is often the case for overseas travelers.
The problem is not simply that iMessages fail. This would be annoying, but the sender would notice and use some other means of communications, such as a text message or even a phone call. The problem is that when the sender has data coverage but the recipient doesn’t, iMessages look like they have been sent, but do not get delivered until the recipient returns to data coverage.
There is an additional nuance that makes this issue even more confusing. In IOS Settings/Messages, when iMessage is turned on, there is an option for Send as SMS, described as, “Send as SMS when iMessage is unavailable…” It would be reasonable to interpret this to mean that when either the sender or recipient cannot use iMessages the message will be resent as a text message. This works when the recipient doesn’t have an iPhone at all. But if the recipient had an iPhone the conversion to a text message only occurs or the iMessage server is down or otherwise inaccessible to the sender. If the iMessage reaches the server there is no conversion to a text message, even when the intended recipient cannot receive it due to lack of data coverage.
The problem is further compounded by the fact that IOS automatically converts text messages to iMessages whenever the sender has selected Settings/Messages/iMessage. Consequently, when the recipient has no data coverage the message goes into a black hole even if the sender has carefully sent it as a text rather than as an iMessage. The fact that IOS converted it to an iMessage does not imply that the recipient is able to receive iMesssages. Earlier IOS versions allowed you to force a message to be sent as a text, but I have not been able to get this to work in IOS 10. The only solution I have found, when you are trying to communicate with someone who can receive text messages but not iMessages, is to turn off the Settings/Messages/iMessage option. In that case, only texts are sent.