[PATCH 3/4] Fix: Avoid release dialing call when call is 0

[Denis Kenzior]

Hi Zhenhua,

> If +CIEV call goes to 0, we may have another dialing call. We
> should not release this dialing call when call=0.

Yes, you're absolutely correct.

I fixed it slightly differently.  The main issue was that it is possible to have 
a dialing and a waiting call under some circumstances.  Neither the current 
code nor your fix handled that case.  The code is much simpler now as well.  
Let me know if the issue persists.

Regards,
-Denis