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. sip.us starting at 1€/month). They will give you a set of parameters to configure into your phone and it will behave like a regular landline after that.