All right folks, finally an experiment that successfully isolates the problem. I powered the phone down, opened it up, removed the 4G module, installed the 2G module, put it all back together and fired it up. Went out on my back patio and wandered around until I got a signal level of 2 to 4 and repeated the incoming and outgoing call tests. With the 2G module, the incoming call registered the correct number in the notification, while the outgoing call hung up immediately when I pressed the “Hang up” button. Left it powered on for 1.5 hours, went back outside, found a signal, and repeated the tests with the same results (i.e.: perfect, even though the signal strength was minimal). Lastly, with the 2G module still in place, I tried the “Force time sync” and it worked perfectly, setting the clock to the local time (EDT).
Then, I again disassembled the case, removed the 2G module, re-inserted the 4G module, put it all back together, and repeated the tests yet again. Immediately after power up an incoming call showed as a blank number with the text “OK” below it and more text “RING” below that. The call log showed “OKRING+CLCC: 1,4,4,0,”, whatever that might mean. The outgoing call test worked this time. I waited about 45 minutes and tried the outgoing call again. The first time I tried, there was no ringing on either the Makerphone or the called phone; I hung up. The second time I tried, I could hear ringing at the Makerphone and the called phone rang, but did not stop ringing until long after I pressed “Hang up”. In other words, the problems listed above came back. Furthermore, I tried the “Force time sync” and it came back 4 hours in the future, just as before.
I believe today’s experiments show that the problem follows the module, not the phone nor its firmware nor the SD card, since the only thing I changed was the module. Somehow this South American version of the module is NOT compatible or is going flakey and a power cycle restores some functionality briefly (though not the ability to properly sync time).
Could you swap this suspect SIM7600SA-H for a known working SIM7600A-H? Then you could test the suspect module at your end to see what its failure mode is, and I might have a 4G module that actually works.
Thanks for putting up with this long drawn out problem isolation - I should have thought trying this sooner.