That would depend on the radio access technology the module is currently attached to, if IMS is enabled and attached, the network and module configuration. In case of 2G/3G the default is CS but it can also be PS. For 4G, it would be either IMS or NAS signaling. If both methods are not supported by the network, CS/PS fallback could be used.
You can switch between text and PDU mode (AT+CMGF) to be used for reading or writing short messages on the application side.
Do you have any particular module type on your mind?