There are four types of MO SMS transport services that can be configured on Minicards and SL80xx modems.
0 Packet Domain
1 Circuit switched
2 Packet Domain preferred (use circuit switched if GPRS not available)
3 Circuit switched preferred (use Packet Domain if circuit switched not available)
Carriers implement one of the above transport services. The default setting is Circuit Switched Preferred which fit most carrier settings.
You can change MO SMS service using AT+CGSMS command. In case you experience issue when sending SMS during active data connection, please change the MO SMS transport service that your carrier is using. In most cases, executing AT+CGSMS=1 should resolve this issue.