Call waiting not working.!

I'm using an Iphone X right now, my elder brother has an Iphone Xs max and my uncle has an Iphone Xr. As we reside in India, our service carrier is Idea. I've tried turning on the call waiting service on all the three phones but an error prompts every time saying "could not save call waiting settings". I've read another post such as mine on the internet (actually on your website only), where apple's team says that its not on their end instead its the fault of the service provider. Then, i tried these same settings by inserting the sim cards on Samsung and a Vivo phones where it works perfectly fine and without any issue.