function in select and where clause - performance

If i have a function that returns a termination date that I want in my insert select source, and I decided to conditino the the functions result in the where clause, will that impact performance with a second call two the function?

Any way condition the results of the function without calling it?

  insert into CAP_CUST_1

      (BTN, STATUS, PLAN_BEGIN, REASON, PLAN_END, BILL_METH)
      select distinct an.mcman_cdani BTN,
                      st.status_group STATUS,
                      trunc(an.mcman_dtfecestado) PLAN_BEGIN,
                      an.mcman_glmotivo REASON,
                      pk_fees.fn_GetTerminationDate(an.mcman_cdani,
 
an.mcman_dtfecestado,
                                                    endperiod)
PLAN_END,
                      st.bill_method BILL_METH
        from mcman an
       inner join vw_status_codes st on st.status_code =
an.mcman_glestado
      inner join cap_cust_testlist t on t.btn = an.mcman_cdani
       where trunc(an.mcman_dtfecestado) <= endperiod
       -- I WOULD LIKE TO CHECK TO SEE IF TERMINATION DATE IS BEFORE A