Jajah is really good. Jajah.com
I use it to call Prague from the UK, costs a couple of pence a minute, or free if both you and the person you are calling are registered.
Just checked out rates if calling from UK to Poland:
From landline: 2 pence to landline or FREE between Jajah Users.
10.5 pence to mobile phone or FREE between Jajah Users.
From mobile phone: 9.7 pence to landline or FREE between Jajah Users.
18.2 pence to mobile phone or FREE between Jajah Users.
* Applicable rates are displayed during the call and may vary depending on the operator.
For users from the EU: all prices are listed without 15% VAT.
As of May 1st 2007, free calls are only available from JAJAH Web.
(Jajah cheap rates still apply to all calls initiated from JAJAH Mobile Web and from JAJAH Mobile Plug-in).
I would check this out thoroughly though because all I know is it worked for calling my folks! I don't really know anything else about the company or even fully understand how it all works!!