When I was first setting up Lync 2010 for VoIP one of the things that really stumped me for a while was Normalisation Rules. In short these rules take human readable numbers and convert them to the standard for international calling (E.164) – its kinda like how computers take human readable DNS addresses (e.g. bbc.co.uk) and converts them to IP addresses (e.g. 212.58.241.131).

So how do we apply this to UK phone numbers?

Well first we have conversion to plain old national calls, here you could take a number like 02682342458 and convert it to (the required E.164) +442682342458.

For this you are looking at a number that is precisely 11 digits long, want to remove the leading 1 digits and add on +44.

Next there are local calls, this is when a user wants to dial someone with only 6 digits because they are in your local town/city/area/ect. Here you would take a number like 236987 and convert it to +441235236987 where 01235 is your local dialing number.

For this you are looking at a number that is precisely 6 digits long, want to remove no digits and add on +441235 (or whatever your local dialing number is).

Please noteCheck with your SIP trunk provider for their details on Emergency calls, the advice below might not apply to your provider and given the potential gravity of 999 calls its best to be sure you have the right settings!

Please also note – Testing 999 calls – It is perfectly ok to test emergency numbers by calling them, just wait for the other side to pickup explain that you are testing your PBX. They may ask for your name/initials but that will be the end of that. However DO NOT dial 999 let it ring and then immediately hang up – you will have them calling YOU just a few moments later asking what is going on.

Then we have good old Emergency Calls to 999 and 112 (EU rubbish), these numbers you can dial direct through without the +44.

Here we are looking for a number that is precisely 3 digits long, you will want to remove no digits and add on no digits.

The last one is a bit of a special case, if you are using the Lync client to use the ‘click to call’ feature (whereby a little Lync calling button appears next to some phone numbers on websites) you may encounter cases were website operators haven’t put their number in quite the correct format and so the leading 0 (which should be replaced by +44) creeps in and results in a failed call.

Short of telling the website owner to get their act together and get their numbers into E.164 you can use a normalisation rule to mask the effect.

Here you will take the +440 from a number that is precisely 14 digits long, want to remove the leading 4 digits and add on +44.

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