Ciao,
ti riporto questa spiegazione (in inglese) che avevo trovato su un sito, e che mi sembra chiarisca bene il funzionamento con un esempio (ovviamente qui parla di Treasuries in rapporto ad un corporate americano, poi bisogna considerare che ogni emissione si rapporta con il proprio "benchmark free risk"):
This call option is used increasingly in bonds and meant to compensate investors with a premium price should the call occur. The premium price is generally expressed as certain number of basis points (spread) over the yield on a comparable Treasury security (T). For example: T+15 bps. One basis point is 0.01%. Let’s assume that a 4.75% coupon bond is trading at par, or $1,000 per bond, and assume that a three-year Treasury note currently yields 1.45%. This means that its yield is equal to 330 bps spread over the three-year Treasury note (475 bps – 145 bps = 330 bps). The issuer may only call the bonds at a yield equal to T+15 bps. In this example, the price of the bond would be calculated assuming 1.60% yield (1.45% + 0.15%) and the call price would be $1,093 per bond.