What you would like to have is not paying your ISP for the landline anymore right?
This is easily done by buying a SIP phone (used Yealink T30 start at 15€) and register a virtual number at a SIP provider local to you (e.g. https://sip.us/ starting at 1€/month). Just connect your phone to your regular network, they will give you a set of parameters to configure into your phone and it will behave like a regular landline after that.
What you would like to have is not paying your ISP for the landline anymore right?
This is easily done by buying a SIP phone (used Yealink T30 start at 15€) and register a virtual number at a SIP provider local to you (e.g. https://sip.us/ starting at 1€/month). Just connect your phone to your regular network, they will give you a set of parameters to configure into your phone and it will behave like a regular landline after that.