always make sure that your phone company does not apply a surcharge for calling the "discount" access number.
Let me give you this example of mine for France (ok we're like 3rd world I know

)
Calling through a 0800 is free, but calling a 0810 (equivalent number in france for a telediscount access number) is not free. These are "special numbers"
To get a "discount" access number from my home, my phone company will charge me 0,078€ upon connection (no matter if the line is picked up or not, or if it's busy) and then 0,028€ per SECOND, that's 1.68 euros per minute ( £1.45 ) and that's on top of the telediscount-like fee
If I use my regular phone line to call to the US, I will be charged 0,25€ for connection and 0,720€ per MINUTE
So, for me, calling the US is twice as expensive using a "discount phone access number" than calling direct.
So always double check that the access number is free for you, and if not, do the maths, cause you could end up paying much more for a so called "free" service.