Hello Jubejubetoot,
Welcome to the community!
How were you notified about the roaming fee charge? When a phone connects to a foreign network, you'll usually receive a Welcome SMS informing you of the Fido Roam charges. However, simply receiving that SMS does not necessarily mean you were charged the roaming fee. As FidoValeska noted, the Fido Roam charge is triggered by usage of the services (receiving SMS does not trigger a charge).
That said, if you were indeed charged for Fido Roam, it's possibly due to some of your apps synchronising when your phone connected, if your data and roaming data were enabled. I understand you might have changed your SIM within a short time-frame after arrival, but data transfer can happen within seconds after connecting to a network.
For the future, it probably best to change the SIM before you arrive at your destination. Alternatively, if you're flying, your phone should already be in Airplane or Flight mode during the flight. Switching your SIM before you disable Flight mode would also prevent your Fido SIM from connecting prior to the switch.
If you do not intend on using any mobile services during your trip, you could enable Flight mode for the duration of the trip. You could manually enable Wifi/GPS when required. Note that if you're using GPS, you would need an offline map.
Hope this helps 
Cheers
Hey @jubejubetoot!
Fido roam is billed when usage is done on the phone while roaming.
Here are a few ways how Fido Roam is triggered:
- Answering a call
- Placing a call
- Using data
- Sending a text message
You can find some roaming tips to help you stay in control of your roaming charges here.
Let me know if that clarifies the charge!
