old cordless phone as a wifi signal repeater/booster . Is it possible ?

Well the question says it all. I am just wondering if its possible

No. Wrong frequency.
Even if the phone was on the right frequency, I believe the answer would be no for a number of reasons starting with feedback and amplifying radio noise along with the signal and signal bandwidth and so on. And that's assuming you could somehow reconfigure the phone to hook its receiver and transmitter back-to-back.