Not sure I understand your question. What kind of phones, and what is the 24V? In the US, the POTS (plain old telephone sets) use -48V for power. And if you are asking if you can use a building ground instead of the 2nd conductor in the twisted pair of phone line, the answer in general would be "not if you want a clear signal". Also, building ground is susceptible to electrical transients (like surges caused by nearby lightning strikes), so the chance of damaging the phones would increase if wired that way.