Thanks for contacting TELTONIKA | Crowd-support forum.
Have you tried theses option?
- changing '+' by '00'
- Rebooting the device
- Resetting your router to default factory settings.
- Trying another SIM card while trying to send SMS.
- Reflashing the firmware through bootloader menu
Issue stayed mystery for some time. Afterwards it turned out to be network problem. Network provider had made some configuration changes for that certain subscriber that we used. They returned old configurations and SMS functionality recovered.
Try troubleshooting steps given in Teltonika answer (particularly "Trying another SIM card while trying to send SMS")