Sometimes you can still receive text messages but not reply to them even if you don't have the service.
There are also instances that you can still receive & send & merely be charged for each one if you don't have a package deal, even though most plans contain a number of texts allowed.
They probably got your number from something you've registered or called.
Some phones or plans give you the opiton of blocking texts from certain numbers, or you can inquire with your provider & they will have a solution.......