Hi buddy,
it is a very interesting problem. :)

Let's discuss it here.

First., you use 'blank' model (e.g. cash flow without mobile phone business) to get basic stock price.
To get there, you need past cash flow statements and discount rate.

Second, you build your incremental model (i.e. cash flow with mobile phone revenue)  to get incremental stock price; also, you need your updated cash flow information, such as, annual revenue, sales duration, etc.   The discount rate is tricky. You may use your former one. Make sure you can provide convincing evidence to do so.

Finally, with two stock prices in hand, you get what you want.


Or,
you may use the following simplified method to verify what your conclusion.

You simply build your model with cell phone business cash flow.
Based on its cash flow and discount rate, you could get a  Δprice.
If it is positive, meaning add value, vice visa.

Just out of my curiosity, what's your opinion?
Thank you and good luck. :)

多谢大牛指点，第一种方法看起来好高深，那种方法用理论解释更为好呢？还请指点啊

Thank you my friend. :)

NPV method is what we used.
A classical way to stock evaluation job done.

The two methods are basically identical.
The load for the first one is heavier, but more straightforward;
The other one looks simply, but you normally don't have enough pure data to conduct your practice, e.g. depreciation, inventory, AR and AP etc.

So, stay at NO.1 may save your time in the long run.
Good luck buddy. :)