cisco 506e and ZyXEL OTP

Hi,

Do anyone have bought ZyXEL OTP and using Cisco 506E?
We are very small company like 4~5 ppl and trying to find a cheap OTP solution. ZyXEL provide server software + Token for 100$. They do said it can work with cisco firewall but it is not confirmed. I am not sure if anyone have already experienced this situation. Or anyone have alternate solution?

Thanks in advance.