Does anyone know how to change BSNL SIP calling account password?

I recently successfully configured zoiper on my phone for SIP calling through BSNL FTTH.
I made the same for my neighbour. But the problem is, I recently connected my phone to his wifi and I could still use my zoiper SIP client on my device.
In other words anybody who can ping the same private IP 10.x.x.x can activate my SIP service on his phone and start calling.
This is lack of privacy as the password is as vulnerable as the phone number. This can be used for malicious activities.
Please answer me seriously

That has to be changed by BSNL in their systems which doesnt reflect on our main accounts sometimes. BSNL is a dirty mess when it comes to accounts and billing type stuff. Same has happened with many people
They change PPPoE user and pass. The thing doesnt reflect itself but the old one also doesnt work sometimes

Thank you very much for your response.
I guess they don't have appropriate TR069 usage capability to make remote changes to our ONT devices.