Hello Naracu,
Welcome to the community!
You are correct, the Fido Roam charge will incur when you use your device to make or receive a call, send a text message or use data while abroad (see here). While receiving SMS would not incur the roaming charge, it's worth mentioning that the data contained within MMS (incoming or outgoing) would be considered roaming data and charged accordingly. You should note that MMS are not solely restricted to pictures or video messages. Messages with subject headers or group messages etc are also considered MMS. In addition, if you subscribe to the Premium Voicemail-to-Text add-on, those notifications would also be MMS.
You might consider disabling any MMS auto-retrieve settings on your phone. You would then receive a SMS notifying of a MMS (no charge) but the data contained would not download automatically. You could then choose whether or not to download the associated data contents. You should also note that, with the exception of Wifi-calling, MMS does not generally get transmitted via Wifi. Downloading MMS while connected to Wifi would still use the networks and likely incur roaming charges. That said, if your device is supported for Wifi-calling, some types of usage would be considered roaming.
As far as I am aware, having calls go to voicemail and/or rejecting calls to send them to voicemail should not incur roaming charges (see here). However, checking your voicemail would be considered roaming as one must call the voicemail service.
In order to prevent roaming, it is generally recommended to enable Airplane or Flight mode on your device. You can then manually enable Wifi when required. However, since a device is not able to connect to the networks with those modes enabled, you will not be able to receive text messages over the cellular networks. If your device and plan are supported for Wifi-calling, you could possibly receive them when connected to Wifi.
With regards to pre-paid vs post-paid, it would depend on your usage preference as well as destination. For example, if you are travelling to the US, you should note that Fido no longer offers pre-paid roaming in the US due to their '3G'-network shut-down. In addition, some features (ie Wifi-calling) may not be available with pre-paid plans. As well, pre-paid services are on a pay-per-use basis, while post-paid services are using Fido Roam (unless you opt-out). You can view the pay-per-use rates here.
If you choose to switch to a pre-paid plan, you should note that there would likely be a fee for migrating to that service type. As well, there would also be a migration fee for switching back to post-paid. In addition, you should also note that if your current plan is no longer in-market when you return, you will not be able to regain that plan. You would need to select a new in-market plan upon your return.
Hope this helps 😀
Cheers
