GSM networks are being closed down in many countries.
Here in New Zealand, 2Degrees have closed down their GSM network, Spark never had one but Vodafone are suggesting they will hold the GSM network till 2025.
Sending SMS messages on an XL4 on the GSM networks, requires that the firmware is equal to or less than 14.16
It appears that SMS messaging on the XL4 on the GSM network has been disabled above firmware 14.16
Firmware above 15.00 seems to support SMS on the 3G network only.
This has been tested on both internal and external modems.