In 1990, the Royal Observatory in Greenwich stopped observing Greenwich Mean Time due to budget cuts. The responsibility for timekeeping was given to the BBC, which then used GPS time to estimate UTC. (GPS and UTC are not the same, as GPS does not admit leap seconds. See http://leapsecond.com/java/gpsclock.htm for UTC, GPS, Loran, and TAI.)

'Z' is a time zone, just as 'U' (Pacific Standard Time) and 'T' (Mountain Standard Time) are time zones. Every clock set to Zulu time -- be it a cesium or rubidium standard, a wind-up alarm clock, a pendulum-driven grandfather clock, a wristwatch, or a sundial -- is designated as 'Z'.

UTC is properly designated only as 'UTC'.

Your cell phone gets time from your service provider, and that time is nearly or equal to network time, which itself derives from UTC, which in the US comes from WWVB, the 60-kHz ground wave out of Fort Collins, Colorado. Network Time closely matches UTC except for leap second insertion, when UTC seconds run 57, 58, 59, 60, while NT seconds run 57, 58, 59, 59, before both roll over to midnight, the next month, the next day, and for December insertion the next year.

Again, GMT ended 32 years ago. Since then, 'GMT' signifies that the person using it is ill-informed or just lazy. GMT was in no way a time zone.

In the movie "Hidden Figures" they have a launch countdown in UTC, but NASA used GMT at least through Apollo 11 (and perhaps beyond).