07 numbers aren't only used for mobile phones. They're also used for 'personal numbering' services, which are expensive to call.
To check the calling charges (from a BT residential landline) to an 07 number, find the relevant prefix in the first column of this list:
http://www.bt.com/pri...31_d0e5.htm#1631-d0e5
Then look at the charge rate alongside it (in the second column).
You can then look up that charge rate, here:
http://www.bt.com/pri...16_d0e5.htm#0016-d0e5
Chris