08979 Oh…



16th October 2018

by Ross McKillop

General Condition 6 requires that all operators sending calls onto the PSTN set a Network Number that is on their network to allow calls to be easily traced and reduce the impact of CLI spoofing which has been a regular factor in consumer harm caused by misuse of the telephone network. Where the call originated without one, Ofcom provided a range of 08979 numbers to be inserted so that the CP that originated the call can be determined and, in most cases, their individual customer. What could possibly go wrong?

Well, quite a bit as it turns out.

Firstly, a thank you… the vast majority of our customers for ensuring they send valid CLI on all calls. With the exception of a few using the long deprecated Remote-Party-ID standard, for which we quickly implemented a fix, less than 2% of our customers had any reason to notice the change.

However, today we’ve received a few tickets indicating that – very occasionally – people seem to be receiving calls with 08979. Thankfully, our customers understand this isn’t our fault – but something wrong in the recipient network or somewhere down the line.

One of the most common networks we’re seeing reports of this from is the UK’s biggest MNO by market share, EE. Whilst we anticipated a few issues with smaller CPs, we didn’t expect one of the big MNOs, owned by the incumbent, to be the worst!

We’ve tried raising this with them directly and initially met with a push back that we “shouldn’t be using 08979” which seems to be a result of a misunderstanding of Ofcom policy, we’ve looked to Ofcom for confirmation here but – somewhat unsurprisingly – they haven’t been forthcoming with a definitive answer. EE were initially getting hung up on the fact that it’s an 08979 number and ignoring the obligations outlined in the General Conditions and ND1016 that they should be showing the Presentation Number not the Network Number to the called party, irrespective of what prefix it has!

Indeed, another major SIP provider has a line in their own CLI Guidelines alluding to issues they had in the past with Presentation Numbers on calls to the O2 and EE networks, but believing they are now resolved yet we’re still seeing reports suggesting this is still happening.

Whilst the pixels on this post were still wet, we’ve since been in touch with EE and this is now being taken seriously and investigated, and we’re confident it will be resolved soon, but it hilights the issue with introducing a change like this when it seems many CPs aren’t quite ready for it!

How do we ‘fix’ this?

This puts us in an awkward position, some customers comment this “only happens with calls over Simwood” well, yes, but that just means that your other provider likely isn’t complying with the rules.

If we don’t send this Network Number, we’re non-compliant, you’re non-compliant, and a call bound for another CP that’s actually enforcing the rules should be proactively blocked when it gets there.

However, if we do send it, it seems that some CPs that haven’t bothered ensuring their systems are compliant presenting a number that should never be presented to an End User.

The short answer is we can’t ‘fix’ it, because it isn’t broken – at least not at our end. These rules apply equally to every CP, if it happens with calls over Simwood it should happen with the same call over any other carrier. Also, the destination network should be presenting the right number!

Of course, you can set a different Network Number, so at least the called party gets something useful.

Are presentation numbers a new thing?

Short answer, no. They’ve been in the ISUP standard since the beginning, and the correct process for handling them when interoperating with SIP has been documented for over a decade in ND1008:2007/01. So this shouldn’t be news to any CP.

Also, ND1016 states;

Presentation Number (when available) is used in preference to the Network Number of the calling or connected party. This is important where the Network Number is not suitable for display, e.g. where PABXs have segregated incoming and outgoing lines.

And this is echoed by the Ofcom Guidance;

3.77 In most cases, the number presented to the recipient of the call should be the Presentation Number. Although some calls will have a different Network Number and Presentation Number, in many situations for legacy networks and in some SIP calls, the Network Number and the Presentation Number may be the same number. Where calls have the same CLI for the Presentation Number and Network Number, CPs must be mindful of the requirements for both types of numbers

It is therefore unequivocally clear that the Presentation Number is what should be being displayed to end users, not a network number.

Why 08979?

We feel 08979 was an off choice for this purpose as it resembles a former PRS code, and would likely cause confusion if displayed to a subscriber directly as many will remember 0898 and 0891. Also, it’s not a diallable number i.e. you can’t call it back!

However, this is a number of last resort, we’ll only send it if you haven’t given us another valid Network Number either in the P-Asserted-Identity header in the initial INVITE, or a Default CLI or Default Network Number on your trunk.

If you’re regularly presenting another number, perhaps an 0800 on behalf of one of your clients that still has the inbound elsewhere, consider allocating a free 03 number to your account that mimics the routing of that 0800, and set that as the Network Number on the calls or the trunk. Then, at least, networks that don’t correctly display the 0800 number will show a familiar format, basic rate, UK number that can be called back.

Playing Catch-up

We shouldn’t have to come up with these workarounds to cater for other networks that haven’t got their ducks in a row yet, but we anticipated this – hence why the option to configure your own Network Number is provided in the API and Portal.

This is a legal requirement, so there’s no excuse for other operators not to be meeting it, and I expect through time everyone will catch up and this will either be completely resolved or become an issue irrespective of who you use for your outbound termination.

As for us; Ofcom, we’ve tried our best in enacting these changes, but it seems that few others have bothered.

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