I suspect your handset settings aren't correct or your sim card has died, is a common thing with all providers. Not sure why it happens as there is no moving parts. Try purchasing a sim swap (costs $5) and call them and ask to swap your number onto the new sim.
Started working fine again from 4:30 yesterday afternoon and still working this morning. Can't see how it could be the sim/phones as it is happening to 2 different phones with 2 different sims at the same times. If one works they both work, if one has problems they both have problems.