Use following steps & secure your Bulk SMS API :
1) Login in our bulk SMS portal For Login visit here.
2) Inside login Left side below corner beside of setting you can get API option click on that.
3) You can get there Security Tab clicks on that. Select there Enable Option. Add there your IP Address & white list it.
NoteIf you have using broadband service for Internet connection. Then your Ip address is changed continuously and you can get 418 this error code. So In that case don’t Enable IP Security Option.
4) If you don’t want API security then you can select Disable option.
Note: you can get Bulk SMS API sample code in developer section under API.