It would not make much sense (from issuer point of view) for the bond to be called at full face value. It would be a windfall for the investor.
There is usually a set schedule of Call Prices and if the ZCB is called in a particular year the redemption amount will be taken from a lookup in this table of values. This RV and the hypothesized call date is what you would use in your formula to value the bond. Often the table is constructed to produce predetermined yield(s) in the event of call.
Details differ depending on the issue
Some Examples
https://definedterm.com/zero_coupon_callable_bond
http://www.reuters.com/article/asia-bonds-idUSL4N0XY39W20150508