We've talked often about termination charges, and how those associated with calling mobile phone telephone numbers are now a fraction of what they once were. It is my understanding that they are around the 0.5ppm mark, with geographic numbers being one or two tenths of a penny per minute and 03 numbers being around about half a penny per minute.

So, all in all, there is now the foundation for call charges for mobiles to be the same as those to geographic/03 numbers, or at least close to. Granted, as some providers have done, having inclusive calls to mobile numbers may be expected to be more expensive than solely having inclusive geographic/03 calls. This isn't necessarily because mobile destinations are more expensive, but that having them zero-fee rated invites the end user to make more calls which costs the provider more.

But what about wholesale call charges?

In practical terms there are only a very small number of network operators, on which many telephone providers must rely. In terms of wholesale line rental, BT obviously serves in 100% of exchanges, with TalkTalk being second behind BT in terms of coverage. Other providers may sell wholesale too.

Crucially, the charges these wholesalers levy on end retailers is going to be the thing that may impact determination of retail charges, not termination charges directly. So do we know how much they do charge?

Openreach, the division of BT that offers wholesale line rental, gives on its website wholesale pricing for line rental, new lines, services such as call divert and 1571, but where are call charges?

TalkTalk Business, the division of TalkTalk that offers wholesale line rental, promotes wholesale line rental and doesn't give any indication as to cost, something we might expect as, I don't believe, it is regulated.

But the point is that the amount these operators charge will bear on the amount end retailers charge. What I'd like to know is whether it's the end retailers holding out on the cost of calls to mobiles, or whether it's the network operators, or a bit of both.

Call-charge differentials can best be seen by those providers that have the model of charging per minute for calls, rather than inclusive calls packages with high 'penalty' rates for out-of-bundle calls.

For example, CIX sells line rental on the BT network and charges 1.2ppm to landline and 03 numbers but 6ppm to mobile numbers. Pulse8, which is on the TalkTalk network, charges 1ppm to landline/03 numbers during the weekday daytime and 0.8ppm at all other times, with mobile numbers being 8ppm at all times. All prices quoted here are inclusive of VAT.

Is it these providers that are charging more, off their own bat, or are they being charged more by the network operators, or is it a bit of both?

Understanding which it is may be crucial in us understanding why they still charge more.