Jacky,
If you are trying to use the analog components to do it, then it is fairly complex to build the circuit by yourself.
Instead, you can try to search the website in Taiwan, they have some mobile communciation product to interface two RS232 products in short range and not too expensive. Then, you just need to add a RS232 interface with your 8051 and done.
On the other hand, if you insist to build the system yourself then try the Atmel and NS site. They have this kind of IC to do the job.
Sunny