A callable bond is similar to an Option-free bond with a call option from the bondholder. It can be thought of as the sale of a call option by the investor to the issuer as it allows the issuer to repurchase the bond from the time it becomes callable until the maturity date. The purchaser of a callable bond effectively enters into two transactions.
Purchase of a non-callable bond for which they pay some price.
Sale of a call option to the issuer for which they receive the option price from him.
The net price paid by the callable bondholder is given by,
Price of the callable bond = Price of the non-callable bond
- Price of the call option.
Value of callable option = Value of an option-free bond
- Value of a call option on the bond.