Sms notifications to landline from other plans

Hi, I manage a few family plans including one for my dad. A few times now I've had text messages from the "sms service" (0333 something or other) either telling me he's been left a voicemail, or to read out a text message that's been sent to his phone.

These notifications from the sms service are coming to *my* landline though! I'm assuming it's because I'm the manager of the plan. Is there any way to redirect them to my dad's landline or better still stop them altogether?

The strange thing is that I don't get all of his text messages this way, so maybe it only happens when his phone is off or if he hasn't read the text himself after a certain amount of time? And it only affects his plan/phone - I've never had this happen on the other three plans I manage.