But let's assume that happens - what is the semantics of such a SIP URI for a) an end user and b) an operator - can I bounce a call to sip:number@bt.com if I find <number> in 4.4.e164.arpa and it returns such a registry-entered sip URI? Can I (do I need to?), as a end-user, distinguish who entered which record? What if I have a IP Interconnect agreement with BT / what if not? I would assume BT might have an opinion here.. Tony?

IMHO this could only work, especially in User ENUM, if this Sip?:number@bt.com;user=phone is used purely to indicate a Service Provider ID (SPID). The domain name given is not resolvable in the public DNS, only within the providers DNS (as proposed yesterday by GSMA with the Local Routing Tables (LRT). One way to clearly distinguish from routable SIP entries would be to use an new enumservice called e.g. SPID to indicate that the domain name has to be resolved by other means. In this case the users may simply ignore enumservice spid because they cannot use it. This enumservise may also be useful in Infrastructure ENUM to distinguish between ENUM entries routable on the public Internet and pure SPID entries (e.g. gsmaworld.com) Richard