Depending on the number of SMS you need to send, you may use SMS modems. Falcom company provides such SMS modem. They connect to the GSM network with a regular SIM card – yes you need to subscribe and you get a SIM card with its own number.
The advantage of such a solution is that it can be embedded. More than this, using a software SMS Server Tools (under Linux), you can choose which modem must be used for a given destination (based on destination number).
Have a look to: