YAY!!! It's so good to see somebody going to Israel!!!
Which program are you with? If you're with Hebrew U (which I was) it would be MUCH better to use their cell-phone plan (CellCom/TalkNSave) because of the in-network with other kids who have the cellphone. I left my regular cellphone at home (Verizon) and kept the number... for some reason it would be more expensive if my family kicked me off their plan for the 7 months i was there.
However, do use Skype when you can- like just under a dollar per hour to use it. The connection in the student dorms is a bit iffy so I always went over to Cafe Aroma for faster Internet connection so I could with people in the US. The internet connection is ridiculously slow but that's definitely not representative of how fast Israel's technology is actually
PS- Drink an Aroma coffee for me, I miss it SOOO much!

If you have ANYYYYY questions, feel free to PM me and I would love to tell you all about surviving Israel!