If I understand you correctly, you need to send SI (Service Indication) message. This is done according WAP specifications that can be found from www.wapforum.org pages.
In real world the message is initated trough Push Proxy Gateway, but in small scale system you can send it using any data enabled phone. You need to build a system that generates the message and sends it trough the GSM modem. To get started with the technology, please see our PC Connectivity SDK documentation.