Voice and Video


Private numbering plans and dual-mode phones

The architecture and role of the PBX is changing, and not even the wise can predict its denouement.

Thus far we are perhaps half-way through the migration to IP-PBXs exemplified by the Cisco/Selsius Call Manager, connecting on the station side over IP/Ethernet and collapsing the ‘switch’ cabinets into more conventional servers and media gateways. Already we see the next wave, loosely termed ‘Unified Communications’, but encompassing several concepts that will again transform the industry. UC is, for many, a slippery term; perhaps that is inevitable during the phase where ideas are tested and refined… but most insiders expect enterprise communications to be transformed over the next few years.

We will undoubtedly see a new debate about how much equipment the enterprise should own and operate. Those of us with the questionable benefit of a long perspective will perceive a repeat of Centrex-PBX options, but it will be much more than that. If we are to realize the potential of new technology, we must question every aspect of the PBX as we have known it over the past 30 years.

Which brings us to the need for private numbering plans. The concept of extensions and 4-digit dialing arrived with the PBX architecture, and brought some advantages, but fundamentally it was about switching a small number of trunk links between a large number of stations. A recent article by Sorell Slaymaker succinctly lists the pros and cons. Many of these considerations are over Aruba’s head, and don’t affect our architecture. However, as a voice-over-WLAN company, we are especially affected by private numbering plans. Since in most enterprises we connect phones over the WLAN to the PBX, they end up on the line side and subject to the private numbering plan. And this can be a problem for dual-mode phones, where everyone likes to dial from contacts.

Contact lists and directories include number strings in a few variations of the public numbering format. In the U.S., we could see XXX-XXX-XXXX, or 1XXX-XXX-XXXX, and for the international traveler +1-XXX-XXX-XXXX, and of course many other international prefixes. These are generally not acceptable to the PBX. In Aruba we have an Avaya server (not running the latest software, but not too far out of date), and it won’t recognize a + at the beginning of the dial string. Even if it did, the whole numbering plan is set up to demand a 9 for an outside line and a 4-digit string starting with 1 or 8 for an extension. Local calls must drop the area code for 9-XXX-XXXX. We could work through this in principle, but as our PBX maintenance is outsourced, the IT group doesn’t want to go to the trouble and expense of disrupting a system that currently works well-enough for them. This is a fairly common situation for a mid-sized enterprise.

But our challenge, as advocates of dual-mode phones, is that even Aruba’s own engineer refuse to learn and dial 10-digit number strings when they are used to dialing by name from the contacts list on their cellphones. We have not found an adequate inducement, even for our geeks: they just won’t eat the dogfood unless they can use their contact list. It was so frustrating to be giving engineers sophisticated dual-mode phones and find they wouldn’t make VoWi-Fi calls on them due to this simple ‘usability’ feature that we eventually caved in and made a feature to fix this numbering plan mismatch.

The solution we developed at Aruba was to edit the dial string between the phone and the PBX. No one really likes to do this: dial plans are domestic creatures, happier in one place than hopping around in different boxes, but we have found that by limiting the digit manipulation to very simple rules – essentially identifying the 10-digit number, adding a 91 to it, or setting up the correct international prefix, we can adapt the public numbering format in the cell phone contacts to find its way out of the PBX. In a perfect world we might want to identify calls to extensions and cut the DID to 4 digits, for instance, but that would mean a lot of administration so we haven’t got to that stage. And we do this by monitoring the SIP exchange between the phone and the PBX, of course: it wouldn’t work for proprietary protocols, but dual-mode phones overwhelmingly use SIP.

As the UC revolution continues, we would like to move away from number strings for addressing altogether. SIP was all along intended to bring many of the benefits of email to the voice world, and a universal voice addressing scheme, whether URI-based or otherwise, would make our lives more straightforward. At Aruba, we are working with Microsoft OCS and looking at Google Voice as well as keeping an eye on Cisco, Avaya and others. There are plenty of new ideas out there, and the rise of SIP trunking and over-the-top UC both offer much promise. For instance the ‘single-number reach’ feature of many FMC systems was unquestioned for several years, but has been subject to some second-guessing and re-examination recently. With a universal addressing scheme, it would eventually be redundant, even though that day could be distant.