Reason is, there are a lot number of 4G phones WITHOUT VoLTE support {and most are NOT ready to upgrade to volte ones in near future} and their Jio4gVoice app is, you all know how it is. So they need 2G voice fallback. Initially it may look attractive to be a BSNL user, but the same thing happened when RCom announced 4G tieup with Jio. People are of opinion that its better to use RCom for 4G (with 3G/2G fallback) than investing in volte phone, but see what is the situation now???
I guess Mukesh makes sure that there will be ZERO advantage for BSNL in this deal.. Let us wait and watch..