Think about how little you lose from telling your folks the great news. True in a phone call they can tell you that they'd rather not hear from you, blah blah blah... but with an sms you can just share your joy and not risk hearing what they have to say about it.
"Hi Mum, guess what, we're pregnant again!! YAY!!!!"
... or something. If they hear it from somebody else they'll complain that you didn't tell them. If they hear it from you they'll complain that you spoke to them, so either way they're going to have a problem. Here's the thing; telling your parents that you're pregnant is the PERFECTLY NORMAL thing to do, and keeping it from them because you're following their rules is the SADLY MISGUIDED thing to do. We all hope that one day in the distant future start they start treating us normally. It starts with accepting nothing less than normal behaviour, and by not letting their rules get in the way of our life.
Write a few drafts out, and think it over?