I wonder if this is just an overzealous definition. Someone pointed out that their phone warned them, but that's something (presumably Google?) built into the software, it doesn't actually mean its a premium rate number (though perhaps still chargeable at 10p or whatever a text would normally cost, so the people with spend caps can't do it)
i.e. it is interpreting all short codes as "premium" because many are, even though this specific one might not be. The bit from BTP about it depending on tariff could be standard blurb because not everyone has text messaging included in their tariff, eg PAYG users who really do pay as they go.
Another example would be 65075, which is the industry standard number that you can send "PAC" or "STAC" to to get a code if you decide to switch mobile networks. It is free to send messages to from any phone, but your phone may still warn you.
On top of the phone warning & spend cap, some providers allow you to turn off the ability to text or call "premium rate" numbers. I wonder if it's falling into this trap, again someone treating any 5 digit number as "premium".
Perhaps this is the sort of thing that Grant Shapps should *actually* be concentrating his attention on?