They hope you will take out a subscription so you can call landlines/mobiles etc... (I did, cheap too!)
And no, there is no catch.... skype to skype is FREE.... I even have it on the iphone and works great.
Another app is "Viber".... for smartphones... family calls for me is free.
Be aware, you are using data when you use VOIP, so if you have a broadband limit be aware you could exceed it.
Skype's a great programme, still the best in my opinion.