Razza, if you allow me to add something for those that will not read the other thread. This is what I posted before;
<Go to Settings-About device-Status and in Signal Strength you will see your real cell signal strength. A signal of -60dBm is nearly perfect, and -112dBm is call-dropping bad. If you're above about -87 dBm, (lower is better) Android will report a full 4 bars of signal, and you showing just 1 bar tells me that you should be getting a very bad signal there. That will never helps you in that department and will clearly explain any battery life inconsistencies.>
One more thing to check is your ASU values on that same screen, see this link to understand about it,
http://forums.androidcentral.com/google-nexus-5/333298-what-your-signal-strength-your-cell-network-provider-your-n5-dbm-asu.html. It will tell you about it, but in a summary, while having lower values in dBm means better signal, it will need greater values on ASU for the same, to have 0 is very bad.
To give you an example, at home, I have good cell signal, not bad, not great, and that means 2-3 bars, but it shows around -91dBm that is good and 48 ASU. And I barely see that ASU value goes lower than 25. So that can tell you that either they have a huge issue with their radios on the S6 or their signal got worst, but then, if you put your Sim in other phone and the signal is better, then it's the radios.