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Making phone call in windows phone 8.1 in a single tap

I'm writing an emergency app and so I need to allow the user to make phone call in a single tap. All the articles I've been reading so far allows phone call in 2 taps. Is there a way to do that in only one tap?

Change app root frame in Windows Phone 8.1 Silverlight application

I have two simple questions since I'm very new to Windows Phone programming:
1:
How do I cange the root frame, or show a different Page on startup, in a Windows Phone 8.1 Silverlight application? I have a LoginPage and then I want to check if the user is logged in show MainPage as root frame and if the user is not logged in show the LoginPage.
2:
Windows Phone Silverlight applications seems so different to me compared to the Windows Phone WinRT applications, or maybe I'm just not experienced enough. For example I can't put a placeholder/hint text in a textBox in Silverlight but it's super easy in WinRT, why is it like that? Isn't that something very basic that many IDE's implements? Or have I missed something mayor when I'm developing a Silverlight application for Windows Phone?
I know that Windows Phone WinRT is used to create universal apps and Silverlight is used to create only mobile apps for Windows Phone, but why is developing a mobile app in Silverlight so much different (at least to me) compared to build a Windows Phone app i WinRT?

For your first question, use the UriMapper.
The basic principle is that you check if the user is logged in or not and based on that, navigate to the correct view.
The code is not all that complicated, there is a very good tutorial up on Shawn Kendrot his blog here...
For question 2, how to add some sort of hint/watermark can be done in several different ways.
But a very simple version is presented on MSDN here... it uses events to set and clear the watermark.

Point 1
You have to go to Package.appxmanifest and under the tab called Application the second field contains the startup page name. Currently it should be MainPage.xaml, just change it to LoginPage.xaml.
Point 2
I don't quite understand your question. Silverlight and Windows Runtime are two different platforms, they have in fact many different libraries. The goal of Windows Runtime is to run the same application on several different devices, so different hardware requirements and different resolutions. It was a need to organize the environment differently and of course Microsoft added some minor changes such as the hint text (e.g.).
What is so different to you? I did the porting of an app from SL to WinRT but it's not difficult nor tricky at all.

Receive calls Windows Phone 8

I am looking for events that I can subscribe to receive notification of call events (outgoing or incoming) on Windows Phone 8. Also if possible try to get outgoing\incoming phone number however I couldn't find any Windows Phone API for doing this. Is there any way I can do using C# or Native C++ on WP8 platform? Is there any Private API I can use to perform this?

One would wonder why you would want to do this..
However for obvious reasons Microsoft does not allow apps to get info such as incoming/outgoing calls. That is why you couldn't find any documentation for it. So the answer to your question is going to be a NO I'm afraid.

Check if phone is in call (WP7)

In windows phone 7 ... Is there is a way to check if the phone is now busy with a call as in android
if yes, what is the name space that serve this?

As others have commented there is currently no API within the Windows Phone SDK to detect whether a call is in progress.
Tim Askins wrote a fairly detailed post on his attempts to get around this but ultimately it just isn't possible.

In windows phone 7

In windows phone 7 ,is there any combo box and table view control.
If any one knows tel , friends.
And how to web service in wiindows phone 7.how to bind those values into list box or table view control.
thanks & Regrads,
selladurai

There isn't a combobox, as such, in WP7 but one of the AutoCompleteBox, ListPicker or LongListSelector (from the toolkit) may be appropriate, depending on your specific needs.
A table or grid view doesn't translate well to the "Metro" style and has usability issues on a small screen. As such, no equivalent control has been provided.
You can use any kind of web service on WP7 but proxy generation from WSDL services isn't directly supported. It's recommended to use a WCF/ODATA backend or a rest based service.
Data retrieved from a web service can be turned into objects and then bound to the UI using viewmodels.
You may want to take a look at the following questions which have already been asked:
Want to start with Windows Phone 7 development
How to start developing for the Windows 7 phone?
Getting Started with Windows Phone 7
Resources for Windows Phone 7 development
https://stackoverflow.com/questions/4318003/learning-to-develop-for-windows-phone-7
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