Last week's episode talked about common errors when setting up resiliency/failover routing. This week, we're talking about...
#2 - External Access Prefixes
A lasting legacy of the PBX era is the concept of dialing an external access prefix to make a call to the outside world. In many deployments, you have to dial a 9 or 8 or something to open a channel to the PSTN. In some systems, when you press 9 (or whatever the prefix is, I'm going to assume the external access prefix is 9 throughout this article), you can hear the change in dial tone as you actually open a connection to the PSTN.
There are many reasons for using an external access prefix, one of the biggest is to provide a clear demarcation between internal and external calls. When dialing "off-hook" - meaning the user picks up the handset, hears a dialtone then starts dialing the number on the keypad - the PBX needs to know where to send the call based on the digits entered on the keypad. This might sound obvious, but if you've punched in the digits 2-1-2-3, the PBX needs to know if you want to connect to someone at extension 2123, or if you're just in the process of dialing an external New York number that happens to start with 2123. If you punched in 2123 and meant to reach extension 2123, then the PBX will connect your call to that extension and everybody's happy. But if you meant to dial New York number 2123334444, then you will be understandably frustrated that you were connected to extension 2123 instead.
The external access prefix deals with this issue in a neat and tidy way. When you dial off-hook, if you enter the external access prefix before dialing your number, then the PBX will not attempt to connect you to an internal extension as you punch in digits. So, when you dial 92123, the PBX won't attempt to route your call to extension 2123, and will instead wait for the user to enter the remaining digits necessary to complete a call on the PSTN.
In many Lync Enterprise Voice deployments, Lync is connected to the PSTN through an existing PBX which gives Lync both access to the PSTN as well as phones on the PBX. Since Lync's PSTN access is via the PBX, then this means that calls that come from Lync usually needs to have the PBX's external access prefix added in front of PSTN-bound calls.
I've seen many deployments where administrators do one of two things:
There are many reasons for using an external access prefix, one of the biggest is to provide a clear demarcation between internal and external calls. When dialing "off-hook" - meaning the user picks up the handset, hears a dialtone then starts dialing the number on the keypad - the PBX needs to know where to send the call based on the digits entered on the keypad. This might sound obvious, but if you've punched in the digits 2-1-2-3, the PBX needs to know if you want to connect to someone at extension 2123, or if you're just in the process of dialing an external New York number that happens to start with 2123. If you punched in 2123 and meant to reach extension 2123, then the PBX will connect your call to that extension and everybody's happy. But if you meant to dial New York number 2123334444, then you will be understandably frustrated that you were connected to extension 2123 instead.
The external access prefix deals with this issue in a neat and tidy way. When you dial off-hook, if you enter the external access prefix before dialing your number, then the PBX will not attempt to connect you to an internal extension as you punch in digits. So, when you dial 92123, the PBX won't attempt to route your call to extension 2123, and will instead wait for the user to enter the remaining digits necessary to complete a call on the PSTN.
In many Lync Enterprise Voice deployments, Lync is connected to the PSTN through an existing PBX which gives Lync both access to the PSTN as well as phones on the PBX. Since Lync's PSTN access is via the PBX, then this means that calls that come from Lync usually needs to have the PBX's external access prefix added in front of PSTN-bound calls.
I've seen many deployments where administrators do one of two things:
- They normalize dialed numbers in Lync to include the 9
- They create a normalization rule where users have to dial 9, but they strip it out, leaving a properly formatted E.164 number, using a rule like ^9(\d{10})$ --> +1$1. They then add the 9 using a trunk translation rule before sending to the PBX.
With #1, this breaks Ken's universal E.164 rule, which states: "NORMALIZE EVERY NUMBER TO E.164!!!" If you don't know what E.164 is, I recommend you check out this post on the topic. For example, if your normalized number for our earlier example is +912123334444, that number is technically a phone number in Chinchwad, US (which incidentally is now on my list of funny-sounding place names). 91 is the country code for US, and 212 is the area code for Chinchwad.
The ramifications of choosing this path is that you have to adjust everything else in Lync to accommodate what you started with adding the 9 to all normalized numbers. This means routes, and even phone numbers in Active Directory. You may argue that this is OK for you, because you have a single site, and you don't have any need to call US. But AD phone numbers can populate users' contact lists on their mobile phones, and they most certainly don't want to be dialing US when they mean to call New York. You could do some additional work to deal with this situation, but it gets messy very fast and doesn't scale beyond a single site. For example, what if you have a second site that doesn't use 9 for dialing externally? If you have to route calls from the first site for redundancy, then you'd have to make all sorts of rules in Site B to deal with how you initially set things up in Site A. As you add sites, this scheme would quickly become unmanageable.
Case #2 is closer to the correct way to do things. Dialed numbers are ultimately formatted to E.164 standards which is good. Since the PBX ultimately requires numbers are prepended with a 9, we can do this with trunk translation rules. Its trivial to add a 9 to every call that routes out to the PBX. However, you shouldn't have to manually create normalization rules that force users to enter a 9. If you recall from the beginning of this article, the external access prefix is meant to help people who do off-hook dialing. When dialing from the Lync softclient, there is no off-hook dialing. You enter the digits, then you press the call button, or ENTER or whatever. Entering a 9 is superfluous in those cases. However, you do want to ensure a good experience when people do off-hook dialing from a Lync desk phone. How do you support on-hook and off-hook dialing without forcing EVERYONE to enter an external access prefix in Lync? Use the External Access Prefix option in the Lync dial plan, as shown below.
This option does several things, firstly it allows people to dial phone numbers in Lync the same way they're accustomed to on the PBX without having to adjust normalization rules. It also makes Lync work the same way a PBX phone works when dialing off-hook. The kicker is that you're not forced to enter a 9 when dialing. You either enter a 9 when dialing from the Lync softclient, or you don't. Either way, the call goes through as expected. Same thing for the Lync deskphone. You're not forced to enter a 9, but if you habitually dial off-hook, then you'll want to, or you'll stand a good chance of reaching an internal extension instead of the PSTN number you dialed.
Rather than rehash what's been said before, have a read of one of my blog posts from waaay back in Lync 2010 days on this topic (don't worry, things haven't changed since then). It shows how to set your Lync deployment up for extension dialing and external access prefixes the no-muss, no-fuss way. It also shows some examples on how the Lync softclient behaves when using an external access prefix.
You can use this in pure Lync Enterprise Voice deployments, or when integrated with an existing PBX. In cases where you connect to a PBX, you still have to create a trunk translation rule to add the 9, if necessary. For more information on trunk translation rules, you can read my blog post on the subject, or refer to Technet.
Tune in next week, where we talk about http://androidtabletgadgets.blogspot.com /2014/10/EVMisconceptionsITR.html" target="_blank">inter-trunk routing, and how people inadvertently set it up without meaning to.
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