That web page and PDF file appear to refer to home (landline) phones and not the BT mobile network.
Mine is set at +447958879879 which is the T-Mobile one. The one you mention above (+447958879890) is I believe the Virgin number.
Are both in use for BT?
BT Mobile and Virgin both use the EE network. I got this info from the BT mobile tech team so I'm presuming it is correct, have you tried with the number I posted?
Please - what is the Protocolfor the BT SMS Service?