Some services like iMessage (& WhatsApp, etc) can work on WiFi alone but may still send a message as an SMS or MMS if an internet connection can’t be found. These messages will result in roaming charges. Similarly, settings like background app refresh, WiFi Assist, cloud-based backups or push emails (or GPS data) can use data and result in roaming charges. If you prefer to use your device on WiFi while travelling, turn off push notifications or options to send SMS messages to avoid these charges. (As well as checking other app settings which may cause roaming charges)
Will I be billed for roaming if I use WiFi Calling to make or receive calls on airplane mode?
Using your device to make a WiFi call or send a text to a non-Canadian number while travelling will incur roaming charges, such as the Roam Like Home daily fee.
If the number you’re calling or texting is outside the destination you’re roaming in, you’ll also incur long distance charges.
When using WiFi Calling, you won’t be charged for incoming calls or calls back to Canadian numbers, including checking your voicemail.
To avoid unintended charges, turn on Airplane mode and use WiFi Calling to call Canadian numbers, check your voicemail or answer incoming calls. This will ensure that you don’t unintentionally connect to a cellular network if you lose your WiFi connection.
For someone like @Sal00 who knows how to use his phone while travelling, his recommendations are certainly valid as he understands the meaning of the term "usage". However, for many people who may not be as proficient in using their phones internationally and the (many) phone's settings, we have the following link recommendations, to eliminate or minimize roaming charges, which discuss utilizing Airplane mode (as recommended in the last paragraph from Rogers' website), or leaving your phone off, or removing the Rogers SIM, or purchasing a foreign SIM, while understanding the restrictions that come with those steps.
So, if you know what you're doing and don't use your phone in the same way you do when in Canada, you should be able to avoid or minimize roaming charges.
* Underlines and Italics are mine.