Why not use
www.voipstunt.com, call UK landlines, USA landlines and many other landlines for free.
I use VoIP Stunt with my ATA, so no need for a PC, in fact with my PAP2 I have Sipgate on one line and VoIP Stunt on another. So far 10 months of free calls to many countries has cost me €6 in total (not €6 per month, just €6.
If you have an 01 or 02 number then the cost for someone to call it from a landline will be exactly as it would be to call any other 01 or 02 number.
From BT on BT Together 1, daytime calls to local and national numbers (01 and 02) are 3p per minute.
Evening and weekend calls to local and national numbers (01 and 02) are 5.5p for up to 1 hour.
If you have inclusive calls packages then the call would be 'free'.