It's a shame that ENUM didn't really go anywhere, but I suppose ultimately many stakeholders consider the phone network being administratively separate from the Internet a feature, not a bug, even though it's now largely an overlay network on top of the Internet itself in its modern form (NGN).
As far as I remember, the Austrian telco regulator was particularly SIP-forward in the early 2000s, and there was a prefix dedicated exclusively to ENUM-based services back in the day.
Unlike its sibling VoIP prefix, which was terminated the "usual PSTN way" by operators that would then bridge to SIP or whatever internally, the idea of the "ENUM first" prefix was that you'd register a number with some registrar and then make it resolve to your SIP client (via a proxy/service provider or directly to any publicly reachable IP address). Reachability from the PSTN was provided via gateways that would translate from circuit switched voice to SIP/RTP, so effectively you could really be reachable for incoming calls independently of any telco.
Unfortunately, as far as I remember both the VoIP and the ENUM prefix ended up being prohibitively expensive to call from many plans (they were billed at higher rates than both landlines and cellphones from many carriers and not included in any flat rate plans).
Still, together with native SIP and Wi-Fi support in many early smartphones at the time, it felt like standards-based Internet telephony was just around the corner, which of course didn't quite play out, and we ended up with the fragmented OTT landscape of today instead that only uses phone numbers as user identifiers, with a per-service privately managed directory instead of a DNS-based one.
As far as I remember, the Austrian telco regulator was particularly SIP-forward in the early 2000s, and there was a prefix dedicated exclusively to ENUM-based services back in the day.
Unlike its sibling VoIP prefix, which was terminated the "usual PSTN way" by operators that would then bridge to SIP or whatever internally, the idea of the "ENUM first" prefix was that you'd register a number with some registrar and then make it resolve to your SIP client (via a proxy/service provider or directly to any publicly reachable IP address). Reachability from the PSTN was provided via gateways that would translate from circuit switched voice to SIP/RTP, so effectively you could really be reachable for incoming calls independently of any telco.
Unfortunately, as far as I remember both the VoIP and the ENUM prefix ended up being prohibitively expensive to call from many plans (they were billed at higher rates than both landlines and cellphones from many carriers and not included in any flat rate plans).
Still, together with native SIP and Wi-Fi support in many early smartphones at the time, it felt like standards-based Internet telephony was just around the corner, which of course didn't quite play out, and we ended up with the fragmented OTT landscape of today instead that only uses phone numbers as user identifiers, with a per-service privately managed directory instead of a DNS-based one.