If the issue is that your phone is 3G, switching the SIM card won't fix the problem. You need a new phone.
We had to replace my wife's old 3G phone last year for that reason. She is a technophobe, at at the time rarely used her phone, so we chose the cheapest phone we could find. However, since getting the new phone she's learned what it can do for her, so it looks like we will be upgrading to something better relatively soon. Created a monster!
A basic principle - there are phones, and then there are cellular networks (service providers). They are different and separate things. 3G is the technology in the phone, and the technology in your phone must match the technology used by the service provider. 3G is rapidly becoming obsolete. If the service provider no longer supports it, then you need to move to a newer phone.
Switching SIM cards simply gets your phone connected as a local user on a network, which bypasses the exhorbitant roaming charges - something that we encounter whenever we go to Canada. I can't believe the prices charged by Rogers and Shaw!
So if you spend a lot of time in the US, having a SIM card for a US-based carrier might make sense. You can just switch the SIM card when you cross the border so that your phone registers with a US provider.
The other thing to keep in mind is that most (perhaps all) of the US cellular service providers now offer plans where the base monthly charge allows for unlimited calling - pricing variations come about as a function of data usage, and if you are like my wife and me and don't use a lot of data (we don't stream Netflix on our phones), you may be able to get by with the minimum data-allowance plan offered by the carrier.
By the way, if you expect to do as many of your fellow Canadians and head south for the winter each year, you might want to look at a different cellular service provider. Some providers offer plans that are designed for folks who move between countries frequently. That approach would eliminate the need to switch SIM cards. Our son tells us that Google Fi is especially good in this regard.
One other thing to consider - most modern phones are now designed to connect to available Wifi systems for both calling and data. What that means is that if you are careful and limit your calling and data access to periods when you have access to Wifi, you really won't encounter those roaming issues. But the key is to have a phone that has that feature, and I suspect an old 3G phone won't do that for you.
So bottom line - you need to buy a new phone.
