Why doesn't BSNL SIP register on native Android SIP dialer but on third party apps?

[@Tx_JK]

Pretty sure of that. But your problem is pretty unusual. I think the problem is in your switch settings.

 

[disablealg]

How can the problem be in the router of 100 other users who are in the same exchange, I mean - they also can't call another FTTH number

 

[@Tx_JK]

Maybe they've configured ONT wrongly. Like I was testing multicast vlan in daily application and the VoIP service stopped registering.
It's that bad.
Or maybe the OLT fault. I don't know.