It maybe worth clarifying with TalkTalk what plan you are on as I assume it's a Legacy plan from Nildram.
Ofcom's advice for 03 numbers is that they SHOULD be included in any minutes package and are charged at the same rate as 01/02 numbers.
From the Ofcom "Call Costs" leaflet - How much do calls cost? Calls cost no more than calls to geographic numbers (01 or 02) and must be included in inclusive minutes and discount schemes in the same way. Calls from landlines are typically charged between 1p and 9p per minute, calls from mobiles typically cost between 5p and 31p per minute. Calls from landlines and mobiles are included in free call packages. - http://consumers.ofcom.org.uk/2009/10/how-much-does-...
I would recommend having a word with them directly to clarify, I believe they now have a NON 0844 number to contact TalkTalk, however I am not sure if this number can be used for Nildram customers as well
Please let us know how you get on.