Develop Reference

How to make a phone call without phone app like google hangout?

I'm not going to use Intent to invoke default system Phone App, such as Google Hangout. Can we make a call or receive a income call inside our own app rather than system phone app?

Yes,You can do that with the help of a VOIP account. Try over internet you can find lot of VOIP providers and you just have to create an account and use the given instructions.

Android provides an API that supports the Session Initiation Protocol (SIP). This lets you add SIP-based internet telephony features to your applications. Android includes a full SIP protocol stack and integrated call management services that let applications easily set up outgoing and incoming voice calls, without having to manage sessions, transport-level communication, or audio record or playback directly.
Here are examples of the types of applications that might use the SIP API:
Video conferencing.
Instant messaging
Check this Example or Example 2

Turn off mobile data for some specific apps

I want to off mobile data for some specific apps,what I want is that if my mobile data is on then I can allow only whatsapp to access,no other app. I know its possible because some apps for same are available on google play but I am not getting any code for the same. I have googled a lot.Any help is appreciated.

Go to Settings > Data usage then you can see app usage and select each app and turn on restrict app background data.

You can make a VPN application. Within the application you can make a list which apps allowed to access internet and which apps not to allow it. Make sure the connection is through that VPN application. Then filter the connection in your VPN based on the lists

You can use VPN, build your own server and transmit all the traffic through that server, than you can filter all the traffic.
Maybe you can google "OpenVpn" or "ToyVPN" in android's sdk for some ref.

Implementing Voice/Video Chat Application in android

my new task is to implement Voice/Video chat native application in Android. I browsed net for the same and it was written that using Android "SIP" API we can do it. I gone through the documents about SIP.In Implementing using SIP we required SIP address for each user which will be provided by different SIP providers. I also checked the SipDemo sample app provided in samples.
Problem is my app will be used by many Users and they can chat with any others.
So, how can we generate SIP address for each user dynamically and establish communication session ?
or each there any other way of doing this ?
Please Help

If you want to test your android app you can generate a free sip account from AntiSip.
and for generating sip account dynamically you have to configure sip on your server with the help of Asterisk:

Two VoIP software can contact each other directly by using direct SIP URI like username#IP:port.
This can work fine on local networks where the UA (sip user agent) has fix address. Otherwise you will need to use a SIP server (This is the main role of a SIP registrar server: to keep a track of the user locations). There are plenty open source software for this like the above mentioned Asterisk but you can also find a lot of services offering free calls (free from softphone to softphone; they charge only if you wish to make outbound calls to landline or mobile numbers).

real-time voice (calling) using WiFi P2P manager on android

I have searched for way to make real-time voice (calls) between two peers but only found how to record voices and send them or sending photos and text. but what I want is for the two peers to be able to call each other. So is it possible that using WiFi P2P manager on android would allow me to create a real-time conversation between the two peers? Does android have that feature?

Wifi p2p is now called as Wifi Direct you can connect with WifiDirect to transfer files between two or more devices you can find the WifiDirect demo project on android developer site
And if you want to work with SIP you must have a valid sip account some site allows free sip account one of them is can be Found here after having a valid sip account you can start calling over wifi and you can download one more open source project sipdroid from Here

Am sure #Ibrahim wants to be able to connect the two android users without a dependency on internet. Using the internet is the 'old' style. Meanwhile do check this library :
https://github.com/fyhertz/libstreaming
There is so much resources here that we are learning from in building connectivity for users here in Africa at large without the need for internet.

Instant Messaging App Mechanism

I've read many inconclusive articles regarding the mechanisms used by IM apps for mobile devices and I'd like someone to please clarify this for me. Do all IM apps for mobile devices simply use SMS? If not, how does the real-time communication between devices work? Is there an intermediate server? Thanks.

Most cellular carriers do not allow sockets which accept connections (servers) to run on mobile devices. As far as I understand, IM services with large numbers of users (e.g. MSN, AIM, Yahoo, ICQ, Skype) rely on a server to transfer the messages between mobile clients. I make this conclusion because each client must log into the server in order to use the IM service; they offer no way to do peer-to-peer connections and avoid logging in to the server.

No, SMS is never used for what's considered instant messaging by most. If you want to develop instant messaging app, you'll need a server and push notifications. I recommend Parse Push, but you should take a look at Google Cloud Messaging, too. If nothing else, it will give you an idea on how this stuff works.