How do I turn sms off

I’m a carer for a mentally ill person, they live a normal life and I use the lebara sim in a gemarc CL9000 phone handset so we can stay in touch.
if the sim receives a sms , it sets a red light flashing on the phone which is disturbing the person so they are turning the phone off at the plug to stop it.
obvs I ring to check if they are ok or need any shopping and the phone is switched off , which puts me in a panic.
Can sms messages be turned off by lebara?