Pricing of "Additional line" replaced with 3G?

[QuantumofSolace]

Hey guys, I'll try to make this quick and clear:

My family currently runs on a family plan with 4 of the 5 numbers being $9.99 (not including text/data packages, etc.)

I am one of those $9.99 lines, but if I upgrade to an iPhone 3G, will my number automatically jump from $10 to at least $70 for phone service? Or is there a lower cost for those "Additional lines" using iPhones than the account holder must pay?

Thanks much.

[QuantumofSolace]

So I've now read that family plans are available and that additional lines ultimately cost $40. This brings me to a different question:

If my Dad, the "main" line on our current family plan, does not upgrade to an iPhone 3G, but I do, will I still be able to take advantage of the $40 monthly cost? Or will I pay the normal $69.99+ costs?

Basically, is the lower $40 price contingent upon purchasing a second iPhone in the family plan, or is the $9.99 cost for additional lines constant (not including data packages), no matter the device?

Sorry for any confusion.

[1ghziMac]

You may have to run into a store and ask that one.