According to a news report on ITV News some 3 customers didn't get the alert, others got it twice or three times, not all devices synced at the same time to get the alert (hence the alarms going at the Crucible at odd times), and people in crowded areas didn't hear it.
It came through on my phone (S23 Ultra) but my Galaxy Watch (also on the 4G network) didn't get it, nor did my Samsung tablet which has 5G capability and was connected to the network at the time.
Work colleagues are saying that they didn't get it when connected to WiFi in the building but did as soon as the WiFi came off.
I'm in Belfast at the moment and quite impressed that roaming handsets (I live in RoI) also received the message, however those on UK SIM cards in the RoI didn't get the alert or those on the IoM or Channel Islands