HI,
Something more that I noticed besides the 32K sim card limitation, is that in my country we use 3 mobile operators. Using the "AT+COPS=?" at-command in a loop, I could see that the SM5100B-D losses the operator network and tries to re-attach/re-register to an alternate operator-network, when I set the COPS selections to automatic. I connected an antennae and forced manual selection and monitored for "+SIND: 11", before moving on...
You should always consider the "+SIND: 11" response before doing anything else...
Also consider that each of the mobile-base-transceiver-stations from the various operators might work at different power levels, so, to ensure RF reception and stability, you must use the antennae on the unit and prevent the re-selection by fixing the operator selection to "manual".
This is quite a nice unit! I love all the cool Operator-NETWORK-info that is available. 
Kind regards
miloSA