Michael,

Thanks for all your great suggestions.
Regarding to the addition of RS232 port for customer to have the choice of their own. No, it will have to be embedded in the product itself, or the product will be meaningless.

To be honest with you, I have never give up the idea of socket modem. However, what I am doing now is to find a cheaper counter part in the far east. Just as you said, if I needed to build my own modem, i will have to spend my time and money on the telcom certs from different country. And that's not I wanted.