a) Add phone to your pc using cable and send content to phones. There are some applications available, which can handle automatically the sending/receiving of SMS-messages and those applications provides also interfaces.
b) Make agreement with operator to use real SMSC (needs more information from the SMSC API (every manufacturers have their own interfaces. Nokia uses CIMD2 and documentation can be found from Forum Nokia web-pages))
c) Make agreement with some "bulk SMS message delivery companies", which are selling possibilities to send/receive SMS. They will provide some external API's and they have (probably) connections to several SMSC providers (read: operators).