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I am using a free caller android app and they restrict me with limited credits. I would like to explore more about how this app works. So I started decompiling the APK file and intercepting networking requests using Fiddler.

I intercepted requests, but none of those are creating the actual call. So after decompiling the APK I understood they are using SIP server for calls. I couldn't intercept any SIP call with fiddler or wireshark.

They are using this SIP server : https://www.kamailio.org/

Varun Kakumani
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Kamailio is one of the powerful SIP Servers and they have good documentation and big mail-user list. You can check their wiki:

http://www.kamailio.org/wiki/

For understanding this case first you should learn "how voip works" subject. You already can not inrecept any SIP call with fiddler or wireshark. Generally, SIP server managers hide their topology for security reasons. Thats why you can not observe SIP traffic to the end of the tunnel. Even Kamailio has its own module that provides hide your network topology with one click :)

https://www.kamailio.org/wiki/tutorials/tls/testing-and-debugging#decoding_of_tls_connections_with_wireshark

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If they can limit your credits, then, of course, they will authenticate your every call and reject it if you don't have enough funds (or out of quota).

So, if you learn a little bit about SIP technology - perhaps, you'll be able to send a call request even without a dialer, but I doubt that the call will be authenticated.

An analogy: if you know a full path to some restricted page - you can paste it into the browser's address field and go there, but if you are not logged in, then you will still get an error like "you're not allowed to read this page".

Yaroslav Fedorina
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    while you make valid points regarding WHY the user stated they wanted to examine SIP traffic, this answer does not really address the specific question being asked. There are a number of good reasons (such as developing or testing a VOIP product) that someone might want to be able to inspect traffic in the SIP protocol. – Chuck van der Linden Apr 07 '17 at 20:14