Hi together,
nice to see that i am not the only one who still sends SMS with these modems 
I expected the same issue - the problem seems to be when sending some batch of SMS (around 10) and receiving answers to them at the same time screws the modem up. Normaly i do AT+CMGS … then the PDU code and CTRL+Z. Then the modem should answer +CMGS: 230 (or some number). But in case the problem occurs, just nothing happens. The only why to get the control back is to send AT+WAC two times.
My “hotfix” for this was to wait about 4 seconds after sending each SMS until sending the next one.
I have the feeling that this is a firmware issue because i use GL6110 and also an older FXT004. The FXT was fine with older firmware, but with 7.47.6 i have exactly the same issue like also the GL6110.