Skype call quality and reliability depends largely on your available bandwidth. If your calls are of low quality or are frequently interrupted for buffering, then your first step should be to increase bandwidth.
You can improve bandwidth by disabling downloads on any devices in your home, including streams and system updates, so that Skype has more bandwidth to work with. Turn off any apps or devices that Skype is sharing bandwidth yet.
All of the VPNs we recommend should give you sufficient bandwidth, but it will help to choose a server located near either yourself or the recipient of your call. This will reduce the time it takes for data to travel between you and the other party.