Why there would be no reason to calculate the yield to call if the bond were trading at a discount to par value?
I read this statement in Schweser note.
Many thanks.
My understanding :
If the bond were trading at a discount to par, the market interest rate is higher than the coupon rate and it is disadvantegous (impossible) for the issuer to call the bond.
Bonds are called usually at a premium, at worst at par.
Why would a company call a bond and pay par or above, when they could buy the bonds on the open market at a discount?
A bond is called only when the market rate is lower than the coupon rate. As the bond is trading at discount, it means the market rate is higher than the coupon rate and any rational issure will not call it back. So, there is no use of Yield to call.
I got it. Thanks all 
My pleasure.