Congrats on the twins! We had to announce it to my parents over Skype as well, as they live in another country. We announced it the day after we found out though, which was coincidentally also Grandparent's day or something. So we just said we'd not been able to tell them something the day before, because it was a Sunday and everyone was busy with church, but we wanted to tell them Happy Grandparent's day!
If it'd not worked out like that though, my EDD is 2 days after my mom's birthday, so I would've worked that in some how. Like oh I've already decided what you're getting for your birthday, or something like that.
My ORIGINAL plan was to send them a box with some children's books in it, and have a note on it to open over Skype with us. But they only get mail every 2 weeks, and we knew that in 2 weeks anything could happen, so we decided to just tell so they could have some good news =)