I use the Siemens N300A IP base with my existing landline and two Sipgate accounts. I'm not sure if Siemens have discontinued it or not but it's still easy enough to get hold of it.I think Siemens offloaded Gigaset as a separate business? I'm guessing this is the product:
https://www.amazon.co.uk/Gigaset-N300A-Station-Answe...
21 years of broadband connectivity since 1999 trial - Live BQM