Rarely seen but exising are those devices where you plug your cell/mobile phone into the home's telephone wiring and use any of your homes ordinary landline telephones to speak and make calls through your cellular provider.
Closest I came to that was a cordless ("landline") phone that allowed use of both a landline and a cellular telephone thourough its one handset. But alas it was designed for only one model of cellular telephone that soon went "out of style".