The way most of mobile savvy people work is that they have bundles of free minutes on their mobile which they use for calling people locally/nationally and then they have VoIP based clients like Skype they use for calling people on similar services locally/nationally/internationally. There is a constant juggle between Mobile numbers and VoIP numbers. What if we were able to use our number with VoIP client so regardless of whom you are calling, if they have a similar VoIP service on their side, you get free call and if they dont have this VoIP client then you use your inclusive minutes or get charged. ENUM will be able to solve this.
According to a Yankee Group report titled "
ENUM Will Be Reinvented as a Strategic NGN Element",
In spite of its early struggles, ENUM, short for Electronic Numbering or Telephone Number Mapping, is well positioned to provide a fundamental underpinning of the Anywhere Network™ as it relates to the efficient routing of any IP-based service across operator domains. It is in this new role that ENUM evolves from its rather meager beginnings to a strategic role in the transition to IP.
Imagine the NHS has 500 telephone numbers that it operates as 0800 freephone numbers to allow customer (patients) to contact various local departments. The cost of each minute of every call is borne by the NHS so ultimately by the British taxpayer. Now the NHS also has VoIP connectivity and decides to advertise their 0800 numbers through DNS using ENUM. Subsequently, every time someone using VoIP decides to call any of those 0800 numbers their VoIP provider will find the 0800 number in the ENUM DNS listings for the NHS and will connect the caller to the medical department using VoIP alone - at no cost to either party (usually).
Clearly with this approach there is scope for financial savings. That said, there remains considerable work needed to achieve even this small goal, let alone the potential options further down the road.
In case you were wondering, ENUM is an international standard being implemented by individual countries separately through their respective Governments. The UK Government, through regulator OFCOM, has assigned the design, implementation and ongoing administration of the project to UKEC who, in turn, have contracted much of the work to Nominet. Nominet administer and maintain the .uk gTLD - when you buy any domain ending .uk it is ultimately sold by Nominet although almost always through a reseller (”registrar”) like GoDaddy.
NRS is a GSMA Managed Service operated by NeuStar. The service facilitates IP interoperability by translating telephone numbers to IP-based addresses. Interoperability is particularly important in facilitating the uptake of emerging services such as MMS, IMS and Packet Voice.
Based on Carrier ENUM, NRS is available to mobile operators, fixed network operators, and related service providers. The service is currently being piloted with a number of operators, with commercial availability scheduled for the autumn of 2008.
As next generation IP-based services proliferate, operators can utilise NRS to position new services behind the telephone number already used by subscribers. Whenever a telephone number is used to identify an end user, the NRS service will facilitate the discovery of URI containing information specific to the service being provided.
NRS is provided as an off the shelf managed service, interoperable on a global basis, providing all the facilities and features necessary to implement an operator’s interconnect policies. Pricing is based on a cost effective “pay as you go” model with no up front capital investment required. NRS thus helps lower the entry barrier for new services and promotes innovation by simplifying the product development and implementation process.
ENUM is going to be hated by the CPs because it will lower their per minute revenue which they are getting at the moment but they it is definitely going to provide new opportunities (and competetion). At the same time the customers will love it because they will get loads of free calls and dont have to worry too much about installing different VoIP clients on their phones. At present it is still in the initial stages with everyone waiting for others to adopt it first but ENUM is here to stay.
Abbreviations:
- ENUM - tElephone NUmber Mapping (I have also seen Electronic NUMbering)
- CP - Communications Provider
- SBC - Session Border Controller
- NHS - National Health Service (in UK)
- NRS - Number Resolution Service
- GSMA - GSM Association
There is an interesting paper in IEEE Wireless Comms Magazine 2008, " ENABLING MULTISERVICE ON 3G AND BEYOND: CHALLENGES AND FUTURE DIRECTIONS" and a gist of that article could also obtained from this presentation by one of its authors. 
This Multinetwork environment providing Multiservice with Multimode terminals is also referred to as Hetrogeneous Wireless Network or HWN.
