Inbound SMS features are not yet supported. It is planned for a future release, but there is no published timeline. Outbound SMS can only be sent to a valid mobile number/account at this time.
culled from the developer forum.
I found this out after attempting to fund my developer account and claiming my $25 credit to allow me test inbound sms to no avail. Apparently, you can only send sms to an actual mobile phone in the US but not to Ribbit's US-issued purpose number. By attempting to fund I mean that I tried with more than one credit card, severally, but with no luck so I put in a support ticket and got my account credited with the $25 all the same.
