This feature is an add on in Skype so I quite surprised when you said it was included, we live in hope.
Skype to Skype is free, but landline and mobile calls arenot
Alexa calling, is alexa to alexa only, this way you can call Skype users, Landlines and mobiles to 150 countries, (200 minutes free for 34 of them)
you can call landlines and mobiles in 34 countries for up to 200 minutes.
Do both parties need to have Skype installed, i.e. if i call from my Alexa Skype to a mobile or landline