The SSML specification does not provide a list of values for the "interpret-as" attribute; it is instead left up to the implementation. You may need to consult the documentation for your SSML processor.
On the other hand, the VoiceXML 2.0 specification does mandate a few builtin types for grammars and the "say-as" element. Note that since "say-as" is actually SSML, you normally must prefix the builtin types with the "vxml:" namespace. You'll need to consult the documentation for your VoiceXML browser to determine whether this is the case. The VoiceXML builtins are:
- vxml:boolean
- vxml:date
- vxml:digits
- vxml:currency
- vxml:number <-- This is the droid you are looking for.
- vxml:phone
- vxml:time
More info here: http://www.w3.org/TR/voicexml20/#dmlABuiltins