"receive the extension number "11" on his CLI device."

Well, you need to generate the CLI in whatever format is recognised by this unspecified "CLI device."

Without knowing your CLI requirements, and what microcontroller you intend using, it's impossible to say!
However, CLI is usually implemented using some sort of modem-like signalling - so that'd probably need some quite hairy DSP. It's unlikely that an 8051 would be a suitable microcontroller for that - more like an ARM!