Again, the limiting factor is the Network - not the modem.
eg, see this site: gsmfavorites.com/gsmhardware/
(my emphasis)
If you need to send large volumes of SMS, then a GSM modem is very unlikely to be suitable - you need an internet connection to a Bulk SMS service provider or “aggregator”…
(although, as Madouc suggested, you might use the GPRS modem to give you the internet link to the Bulk SMS service)