Sorry for the delay.
Had a chat with our SME and these are my notes.
If on-premise the mp124 connecting back to a SBC will work.
But what version of Skype for Business have you got?
For Cloud only Skype for Business or Teams
Purely Cloud Skype for Business- install a SBC, cloud connector edition.
Pros - works.
Cons- expensive, the CCE Edition is 4 or 5 VM's that need TLC as per Sonus, so a support contract needed, plus a good vendor with a damn good engineer.
Teams - if Teams then an entry level SBC with analogue ports will work
Internet Modems won't work
There is a potential fax modem won't work on some solutions.
Ideally keep the POTs line!
This last line is his best solution...