DC adapter power output equation
To replace your cordless phone adaptor, you need an adaptor rated for 9 volts, and 500 mA (Not mAh!) or more.

The adaptor voltage must match the phone's requirements, but the adaptor's power or current rating may be greater thant the phone's requirements - the phone will only draw the current it requires.

(mAh is a specification for the amount of energy that can be stored ini a battery, and has little relation to the current the battery can deliver.  It is meaningless when used for power supplies.)