I've been scratching my head a bit here, about what I should do.
Current config is normal BT landline, with DSL on it, which is now not used.
Internet is with AAISP, but connected via L2TP tunnel from my Firebrick through a Starlink connection.
The analog part of the landline lands on a TDM400 card in an Asterisk box, and hence to IP phones.
I wish to keep the landline number for now, for a few reasons.
My current plan is this:
-Port the number to AAISP VoIP service.
Yes, they can maintain the ADSL service on the copper line if required, but I think I will just allow the entire line to cease that as it's now un-used.
-De-commission the Asterisk box, in favour of using the VoIP gateway functionality in my Firebrick.
It has enough of the features of Asterisk that I need: it will accept registration of the phones, and will register with the AAISP gateway for external connectivity on my ported-in number.
-I will need to provision TFTP which is running on the Asterisk box elsewhere, because the Cisco IP phones pick up their config from it. That can be moved to my Roundcube webmail server.
This needs to be thought through a bit more, but that's my general direction.