Hello  @JoHanstead ,

I think terziyski has already given you comprehensive advice. 

  @JoHanstead 

If you don't want to use a DECT phone just don't plug that in your ADSL splitter.

If the modem model you are referring to is VR2100v and you won't use a physical device like a DECT phone just don't use the back RJ-11 ports (Phone1, Phone 2).

• For Residential Users: Phonely utilizes their own dedicated mobile application called the Phonely Call App (available on both iOS and Android). For home phones, they utilize plug-and-play digital voice adapters.
• For Business and AI: Phonely’s business and AI phone services utilize their proprietary Phonely AI platform, which powers built-in AI agents, workflow automation, and custom call workflows

In fact if you're not using a DSL service from your ISP and don't need a DECT phone, you can get a normal wireless router for the purpose.