If you can manage to connect to UMA while in the US, you will be charged for any calls as if you were connected to UMA at your house billing address in Canada (i.e. daytime calls will be deducted from your bucket of minutes, long distance calls will be billed as if you made the call from your home, and if you have a UMA plan then you will just get charged according to your plan). So this typically means you can call back to your local calling with NO roaming fees.
The tricky part is connecting to UMA while in the US. It seems to be hit or miss.
