By generating frequencies with software, you are going to save a few dollars. Usually this is done in the projects where large volumes are to be produced. But since you have to produce only one unit, I recommend you not to go for this.

I assume that you want to generate FSK signals for CLI information to be sent to the extentions. Although I haven't generated the frequencies you mentioned, but once I implemented DTMF dialling using the two port pins of the 8051.

As you know that 8051 will generate square waves, so you will face another problem of converting them to sine waves which wouldn't be as smooth as a CLI generator chip would deliver.

I recommend you to visit this website for a suitable chip

www.cmlmicro.co.uk