Recently I had problems connecting to Skype on my old Lumia 900 and had a vague suspicion. This was confirmed when I read an email from Skype today telling me that "Skype apps for Windows Phone 7 are permanently retiring".
In other words, if you want to use Skype and you have a mobile phone running Windows 7 - too bad! You can now only run the Skype app on a Windows Phone 8 or 8.1 device. Or you can switch to a Android phone or opt for the Apple iPhone.