Can my parents read my google voice texts through our phone company?

Because I have verizon and I dont have unlimited text messaging, I got Google Voice so texting numbers that werent on our plan wouldnt cost extra. I also got it so my parents wouldnt ever be able to read my messages. I just recently found out that they read them. I have no idea how, but they said they got them from the phone company. If this is true would someone please tell me? This is actually way more important then it sounds.|||yes