As above it sounds like you need to lock out your firewall to only accept traffic from your SIP provider
Out of interest is the router you’ve daisychained the one you were using with your old ISP? Have you cleared all the connection settings / disabled DHCP etc so it’s genuinely just acting as an extender.
Do you get the same issue if you disconnect the daisychained router and connect the phone direct to the BT router?
Reason I ask is that I had issues with one of our home users last year who inadvertently had two ADSL connections from two different providers (Sky and Eclipse) live on the same line! Not sure that’s possible with fibre however.