in this ac is converted to dc and then converted coverted back to ac.i found a component RDN240/20 which is a varistor ..i like to know abt its availability

 

The RDN240/20 is a MOV - Metal Oxide Varistor, that clamps at 240v.
You don't have to use that exact part; just a MOV with a similar rating.

That design is not very good, as there is no "real" regulation. It will not be very efficient.

Since I don't know where in the world you are, suggesting a vendor is difficult. People in Europe tend to use vendors like RS Components, Farnell and Maplin.

 

Interesting schematic, similar to one I found a while back, but I can almost guarantee you he's located in India.

Why not just add a backup battery inside the phone base or purchase a regular one in case of power failure?

 

The worst thing about that circuit is the battery connected straight to the rectifier output. If it is fully charged and the mains is a bit higher than usual, it will be toast pretty soon.
Diode with a limiting resistor in parallel on the input side could help a bit.