Cell phone madness

Is it less expensive to:

A) Call a Canadian cell phone that's being used in the US with a US cell phone that's being used in the US;
B) Call a Canadian cell phone that's being used in the US with a Canadian cell phone that's being used in the US;
C) Call a Canadian cell phone that's being used in the US with a US cell phone that's being used in Canada; or
D) Call a Canadian cell phone that's being used in the US with a Canadian cell phone that's being used in Canada?

Anyone who can come up with the correct answer wins a touque!