Emojis are Unicode
It could be trivially done by the device sending an SMS to some number which it knows, or can obtain via some protocol embeded in the Android O/S. The system receiving the SMS grabs the CLI and returns it to the device, via SMS or the internet.
At the device vendor level, there is a special SMS type called something like “network message”, which can be hidden from the user. For example a non-jailbroken Ipad has its SMS functionality blocked from the user interface by Apple, but it can receive and display (and act on, if required) these special SMSs – examples. Old Nokia phones could transmit such SMSs, though I think that needed a cable and some PC software.
There are also “flash messages” which are SMSs that pop up without being stored in the Inbox. I have a program at work which can generate them, over an RS232 (DLR-3P) connection to a 6130i 
So anything is possible
Peter wrote:
The number is not in the SIM card. It is associated with the SIM ID by the network.
I am not exactly sure how it’s done, but my Android tablet displayed the phone number of every SIM card I stuck in it. It does not necessarily mean it’s true for every SIM card in the world, but it’s at least a popular optional feature.
I understand that text messages with certain emoticons (basically any type beyond the basic ones e.g. :) ;) :-) :P and a few others) are sent as MMS
Not on an iPhone at least. I just tried it. No data connection whatsoever (GSM-based or wifi), yet still able to send a full range of emoticons, including ones that have only been developed very recently, e.g. people with different skin tones. Apparently SMS uses UTF16. That does mean it won’t be able to send everything – some characters these days use four bytes.
The number is not in the SIM card. It is associated with the SIM ID by the network.
You must have at some stage entered it in, for each SIM card, maybe in some context like in your owner’s details and then it would get picked up for any SIM card, as soon as the phone calls up Apple, which it does at least every day.
The iPhone gets its number from the SIM card, nothing to configure manually. I swap the SIM in mine when traveling between the US and the UK and the phone always knows which number is active.
I understand that text messages with certain emoticons (basically any type beyond the basic ones e.g. :) ;) :-) :P and a few others) are sent as MMS and therefore sent over GPRS/3G/4G. I’ve noticed that the modern phones have a massive range of emoticons – here is just a tiny selection
and I don’t think they get sent as a “character”, not even a 16-bit one.
It could be that the phone falls over to mobile data (and sends the message via MMS) if it can get LTE but not GSM. Not sure however if it is possible to have LTE without GSM…
Peter wrote:
But there were no signal bars showing at any time
The bars are an average over time. So it’s conceivable that you might have a signal at the moment you press send, but not show any signal bars. That doesn’t explain why you’d see wifi activity at the same time though. MMS uses a data connection of course, but I doubt it’s using that.
Three have a similar app called ‘inTouch’ which allows SMS and calls over wifi but in the UK only. I wonder if routing through a VPN or something would be enough to bypass this though…
No signal bars is a weak signal. A circle worth a line through it is no signal.
