Q: I am from india. I have iphone 5s. call merge option is not working in my phone.

Is call merge supported by your wireless provider? What did they

say when you contacted them?

To add to imobl's response, with which I agree:

Is your cellular service GSM or CDMA?  If CDMA, the likelihood it supports call merge drops dramatically.  See "Conference Calls" here:

Call forwarding, call waiting, and other call features on iPhone - Apple Support

Also see this page from the iPhone User Guide:

https://help.apple.com/iphone/9/#/iph3c99551d