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How to make full screen caller id in android?

I Want to develop Full Screen caller application with Caller details and accept call and ignore button how to achieve this? .I searched a lot but i'm not able to find solution or any reference, so anyone can lead or help me or any reference it would be helpful for me, thanks in advance happy coding.

I'm not going to give you ready-made code even though I could probably dig into my own and do it. You said you searched a lot but if this were true, you'd have found many references to articles and docs such as these.
Get contact data from number
Take control of calls programmatically
Detect incoming calls
Interesting considerations for incoming calls
That should get you started. But it should go someting like this: Register that you want to be notified when incoming calls are received. On that notification you receive launch your own activity that takes control of the calls programmatically (button to dismiss, button to accept?). That screen can show whatever you want, beit the contact information you retrieved from the phone number, a photo, etc.

Hiding the phone number when calling from iOS or Android App

This might not be an appropriate question for StackOverFlow. If not please recommend me where I can post this question.
We are in the process of developing an application where the customer can call the vendor and the vendor can call the customer. But for privacy reasons we don't want to expose the phone numbers of the customers to the vendors and vice versa.
What are some recommended solutions for this problem?

It's not possible, the URL Scheme requires a number and it is show on the screen while calling.
as you can see here
Maybe, you may have to create something more complex like Skype.

Unfortunately that is not possible as of now. You could hide the phone number by replacing each number as an asterisk (*) in your app.
However, when vendor or customer taps on the phone button to call, then phone number is passed to the device Operating System to call, then this number will show up in the calling list.
In addition to that, your app most likely will not be allowed by Apple reviewers, because user needs to see what phone number he is calling. He might be punished if he keeps calling this secret/scripted phone number which might be 911. That is another serious issue may rise.
My recommendation is to have chatting system, or implementing VoIP(voice over IP) like whatsup or viber style calling within the app. It requires exhausted implementation.

Preventing a message from being sent in android

I know I can detect a message that is being sent using the following code
ContentResolver contentResolver = context.getContentResolver();
contentResolver.registerContentObserver(Uri.parse("content://sms"),true, myObserver);
what I need to know, is it possible to prevent it from going out or suspend it?
Thanks

Not possible unless you are the Messaging application that the user typed the message in to (then you could do whatever you want with it, including not send)
However if the user is using the stock Messaging app or another 3rd party one there is not a way for a different application to try to stop the SMS from being sent (this ability would almost certainly get abused.)
I think there are apps meant for use when driving that will block incoming texts from being shown to user, maybe look in to those, if there is a way to do outgoing maybe they'll have figured it out. I don't think it is going to be possible on a stock device though.

Can you send a message to clients who have just joined your access point?

I am wondering if there is a way to send a message to users that have entered your wifi's reach? I am not sure if this is possible or how it's called.
A hint of what to look for or some sample code (if this is possible at all) would be much appreciated!

The only thing that is possible as far as I know is to add software to your accesspoint to do a splash screen (or possibly a login page).
This would not completely solve your use-case, but like the various "free" wifi in bars you do need to log in to first, you need to open a browser first and perform an action, before you have a real connection.
You could just skip the authentication-part, and put your message on the splash screen.
A random example of what you can find on the internet is this: http://www.dd-wrt.com/wiki/index.php/NoCatSplash
I have no experience with it, but it seems to serve above mentioned purpose, and you can easily go from there.
I am aware that this is NOT an android specifi answer, but as far as I know it is not an android specific question: you want to send a message to people connection to your wireless.

Reading outgoing emergency numbers

I have a fantastic idea for an Android app but I would need to be able to detect when an emergency call is made from the handset. I only need to be able to read the number, I won't be changing it at all.
The documentation for NEW_OUTGOING_NUMBER says:
Emergency calls cannot be intercepted using this mechanism, and other
calls cannot be modified to call emergency numbers using this
mechanism.
Obviously I don't want to find out if I can read an emergency number by dialing it. I was hoping someone here might be able to tell me whether or not I can grab EXTRA_PHONE_NUMBER if the number would be, for example 999.