@Surya, this can be because of a few reasons-
- SIM compatibility issue- it won't be the first phone with that problem. Sometimes older SIMs are known to cause problems in newer phones. To cross this out, please try with different SIMs, or by upgrading your SIMs.
- A compatibility issue between any call enhancement tool (maybe a volume control app) which you had installed before upgrading the OS to 7.1.1. A factory reset may help here.
- Actual hardware problems- these can range from problems with the speakers or mic (which are not expensive to repair) to issues in the antenna band or the chipset (which are hard to repair outside).
The best solution, if Smartron's service centres are not nearby- is to show the set to a knowledgeable and trusted technician who can at least help diagnose the problem. Once you know the problem, then you can take an informed decision about what to do. Holding on to a faulty handset will do no good.
In any case- I suggest you keep an old phone as a backup, always. These handsets (whether Smartron or Samsung or Apple) are complex and made to a price (doesn't matter what they charge the customer)- so defects or problems can happen. I've known centres of known manufacturers reject claims of top line phones that had problems out of the box- so anything is possible.
Better to be careful than permanently inconvenienced.