I have a US plan - 1400 minutes, unlimited mobile to mobile - $99 month, 2 phones, 2 numbers. I also have a usb modem connection, unlimited at $80 a month. The only way for me to use my Verizon cell in Canada (besides having to pay 40 cents a minute on calls to the US) is to sign on to the Canada plan for Verizon which is 1400 minutes, 2 phones, $109 a month, but nothing is free or unlimited for the Canada based phone, including calls to the other cell in the plan in the US. Every call I make from Canada goes against the 1400 minutes - no exceptions. US based calls are also part of the 1400 minutes, but the US phone (I have 2 cells - one me, one for my wife, who will be staying in the US) keeps unlimited nights and weekends and mobile to mobile.
Her calls to me in Canada will be against the 1400 as will my receiving the calls, so I am burning two minutes up for every minute I talk to my wife - there is a joke in there somewhere!

As far as keeping the same USB modem, it's free but the only deal they have is 0.01 a KB - nothing else. So it does not seem worth it to downgrade my US cell plan to the Canadian - I might be able to get by on the Internet charge, as I am mostly interested in email and MSOffice attachments to the email - I have to figure if a Skype call burns the KBs. It may be easier to buy a pay as I go cell phone up in Canada - I think my rate then from Canada to US will be 13 cents a minute - I can live with that for a few months.

I really have to say that I think Verizon sucks with this type of deal.