All HMRC helplines are now 0300 or 0345.
All DWP helplines are now 0345 or 0800.
All central government departments, their agencies and private partners and most other public services have changed to 01, 02, 03 or 080 numbers and no longer use premium 084 or 087 numbers.
Check their official website for their current numbers.
Calls to 03 numbers are inclusive from landlines and mobiles, else charged at 'geographic rate' - the same as 01 and 02 numbers.
Calls to 080 numbers are free.
If you are calling 0845 numbers you are calling either the old numbers or have been caught by a premium 'call connection service' scam.
If you are calling 0843 numbers you have definitely been caught by a premium 'call connection service' scam.
Section 5 and 6 of this article may be useful:
http://forums.moneysavingexpert.com/showthread.php?t=5360819