Hello Kelly72,
Welcome to the community!
Unfortunately, there is no way to adjust a plan to only receive text messages. Fido does offer a temporary suspension of services, however, you would not be able to use any services, including receiving text messages. The Fido Roam charges will incur if your phone uses services -- make or receive a call, send a text message or use data abroad.
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. Disabling MMS should prevent that data transfer. Though, it should further be noted that iMessages and RCS messages are sent (and received) as data and not as text messages (SMS). Even if the text message received is SMS, if your device is configured to provide Delivery and/or Read receipts, those would be sent as SMS or data and considered roaming usage.
Unanswered incoming calls and having them go to voicemail should also not incur roaming charges (see here and here). However, it should be noted that checking your voicemail would be considered roaming as one must call the voicemail service. In addition, it should also be mentioned that the Voicemail-to-text feature notifications would be sent as MMS (roaming data). Similarly, I believe iPhone's Visual Voicemail would use data (roaming, if abroad).
In order to prevent unintended roaming charges, it is generally recommended to disable data and roaming data. However, you should note that disabling those settings does not prevent roaming for voice and text messaging. In addition, some manufacturers might allow some data usage to occur despite those settings (see here). That's why it is also generally recommended to enable Airplane and/or Flight modes in order to prevent unintended roaming usage. You could manually enabled Wifi when necessary. However, enabling Airplane mode would prevent the device from receiving text messages over the celluar networks.
Unfortunately, if you wish to receive text messages, there is no 100% guarantee your device won't use other services. The mobile providers only know usage. If your device uses additional services, that usage would be considered roaming.
For completeness sake, it should be noted that while using Wifi for regular internet access will not incur Fido Roam charges, there seems to be some misunderstanding regarding Wifi-calling. I'm not sure how people have gotten the impression that Wifi-calling does not use the networks. However, that is not true. While the calls and messages do not transmit via cellular towers, they still use the networks via the internet gateway (see image here). The cellular towers and Wifi are merely alternate means of accessing the networks. Calls and messages would not be able to complete or get sent/received without the networks. As such, using Wifi-calling is technically using Fido services. However, they have allowed certain usage to be free from additional charges. All the messages and calls you receive (from anywhere in the world) will be taken from your plan's voice minutes and messaging limits. As well, all Wifi-calling messages and calls you make to a Canadian phone number while abroad won't incur long-distance or roaming charges (see link above).
Fido Roam is enabled by default on most current post-paid plans. If you did not wish to avail of the feature, you would have to opt-out of Fido Roam. In doing so, any usage would be on a pay-per-use basis. Those rates can be found here. You should note the forums are community-driven and not intended as a venue for customer services. If you wished to discuss your matter and/or opt-out of Fido Roam, you would need to contact customer service.
Hope this helps 😀
Cheers
