So do the companies purchase one year call option or the length of the annuity call options. Do they repurchase options every year, if so, how do they reserve the amount needed to purchase future calls?
This will vary depending on the crediting method(s) chosen by the client.
One thing to note though, is that call options that match the length of most annuity contracts don't exist. Most call options expire in 3 months. Some contracts, called LEAPS, last a year plus, but I'm not aware of any that last much longer than that (aside from options granted in ESOPs to employees, and those can't be purchased on the secondary market by insurers anyway).