I have been investing in bonds for over 40 years now. I have never seen a fixed rate bond that becomes a variable rate bond after it becomes callable. However, I could certainly believe such a bond exists.
What I have seen is bonds with a sinking fund. Here the debtor must redeem a fixed percentage of the bonds. The bonds redeems are selected at random so
to the holder of the bonds, it is a lottery and if your bond is selected then
your bonds are called in.
I also remember the days where callable bonds would have a call premium with 3% being typical. Hence if they called the bond early, you got a premium. Today, I am not seeing a lot of bonds, with that kind of call premium.
I have seen bonds where the interest rate increases over time. The idea here is that if interest rates rise then the buyer is protected. I have never bought such a bond because they typically have bad call features. However, the increasing interest rate does protect the buyer if it is not called. From what I have seen, these bonds tend to be called a few years after they are issued. When I am buying a bond, I would like to buy a bond that I can keep for 8 years or more.
I feel that a call feature, to the buyer of the bond can be described as heads your lose, tails they win.
I am in the United States and the rules could be significantly different in your country.