I’m using a spitz (GL-X750) with an AT&T MVNO SIM in the US and in the system log I see messages every 10 seconds of the form:
Wed Feb 6 16:25:49 2019 daemon.info smsd: 2019-02-06 16:25:49,3, GMS99: Error: Unexpected answer from Modem after +CREG?, waiting 1 sec. before retrying Wed Feb 6 16:25:50 2019 daemon.info smsd: 2019-02-06 16:25:50,3, GMS99: write_to_modem: error 5: I/O error
The modem is connected and I can communicate using it. I also noticed that if I run a ping every second to some remote host I get virtually 0% packet loss:
Max rtt: 320.037ms | Min rtt: 0.717ms | Avg rtt: 185.967ms Raw packets sent: 335 (9.380KB) | Rcvd: 334 (9.352KB) | Lost: 1 (0.30%)
but if I run a ping every 5 seconds only I get significant packet loss:
Max rtt: 275.849ms | Min rtt: 0.720ms | Avg rtt: 40.461ms Raw packets sent: 19 (532B) | Rcvd: 13 (364B) | Lost: 6 (31.58%)
Is there an idle-timeout of < 5 seconds?