What version of the plugin are you using?
On the bulksms api site it says a non-200 error should be considered transient, try again later!
But actually I think that it is not working because you need to be using a SA bulksms EAPI address
The plugin currently gives you the choice of
http://community.bulksms.co.uk/eapi or http://bulksms.co.uk/eapi
I suspect you need to be using a South African server. Could you login to your bulksms account and goto your profilepage , which will show you the “EAPI URL” and post it for me.
I’ll then tweak the plugin to give that as a choice and then we can see if it is working!