on 01-09-2018 18:41
SMS uses the mobile network, not data of any sort, so will still send even with no data connection as long as you have a mobile signal.
In the best tradition of the forum and side-conversations, I thought you might know this, @MI5: SMS, does the network attempt retransmission if the receiving number is off the air? Or is the message "lost" ?
An ex-Ericsson engineer at my work says it is like the UDP protocol for computer networks - a message sent is "fire and forget", no acknowledgement necessary by target phone. However, I think I have seen instances where a burst of messages lands after a switched-off phone has been powered back on (within limits).
Just thought you might know offhand, if not I shall ask the Oracle, aka Google.
on 01-09-2018 18:50
If no handshake, the gateway will continue to attempt to send, at first this is at quite frequent intervals, like each 30 mins, but then the gap gets bigger as time passes with no handshake. From memory, it will continue to attempt to send for up to 30 days.
The same process applies to MMS but the retry period is only 7 days.
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone 8+ (EE)