Hello Saaber74,
Welcome to the community!
I assume you're referring to a 3Gb data-only plan. They are considered data-only because the plans do not include any call minutes or text messages. However, since they have a phone number associated with them, they are capable of making/receiving calls as well as sending/receiving messages on a pay-per-use basis.
You will not incur any extra charges when you receive text messages. However, sending SMS costs 25¢ per message sent from Canada to a Canadian wireless number or 45¢ per message sent from Canada to a US or international wireless carrier (see here).
Sending MMS to a Canadian wireless customer costs 50¢, while to a US or International wireless customer cost 75¢ (see here). Again, you will not incur any extra charges when receiving MMS while in Canada. Note however, roaming data charges would apply when outside of Canada.
In addition, you should note that you will be charged for any calls you make or receive. The rates are $1 per minute for your calls and Long Distance (LD) calling at 45¢ per minute within Canada (see here).
Any calls you receive will be considered local if you are within your incoming local calling area (LCA) -- regardless of where the calls originated (internationally or domestically).
Your incoming LCA is based on the geographical location that is associated with your number. It has nothing to do with your own physical location at the time you receive a call. On the other hand, your outgoing LCA is determined entirely by your physical location at the time of a call, and has no ties to any specific geographical location, other than the one in which you're standing.
However, receiving calls while outside of your incoming LCA is considered LD -- regardless of where the calls originated.
Did you ask Fido to block all messaging on your account(s)? I do not believe it would be possible for them to block a single number. Otherwise, incoming messages would have to be blocked from your device. Depending on your device, there may be options to block your device from messaging that number.
I understand you have not messaged that number yourself. However, some apps may periodically send messages without the device owner's knowledge. You might consider going through your apps and app permissions to see whether any apps have access to SMS.
Alternatively, if you do not intend to use any calling or messaging services, you might consider asking Fido to block both of those services on your account.
Hope this helps 
Cheers
Thank you @Cawtau for this detailed reply!
@saaber74, I wanted to take a moment and Welcome you the Community!
I can totally get how it isn't a pleasant situation to receive a bill that is higher than expected. We would hate to you see go! I'll send you a PM so I can take another look into this with you in details.
