Welcome to the community!
As FidoJulien has mentioned, incoming SMS are not charged. However, you should note that any data contained within a MMS (incoming or outgoing) would be considered roaming data and charged accordingly. FidoJulien has suggested ensuring your data and roming data is disabled to prevent inadvertent data charges. You might also consider disabling any auto-download settings for MMS. You should then receive a SMS notifying of a MMS (no charge). You should also note that MMS does not generally get transmitted via Wifi (with the exception of Wifi-calling) so downloading MMS while connected to Wifi would still be considered roaming data (cellular data enabled).
Hope this helps 😀