There's also
8) the phone line to skype voip bridge
9) a static phone number for your voip that you can answer anywhere in the world
10) reasonably affordable subscriptions for the phone line to skype bridge.
11) Simple hardware you can use as a phone by simply logging into to skype
I know there's alternatives, but it has served me quite well for several years and changing means having to buy new VOIP phones.
Skype has its warts, huge ones, like very misleading errors.
Just today I got an email: "Your Skype Number will expire on" even while it has auto renew set to on.
Then after login it says "your skype number has been cancelled". No it has not been cancelled.
What ended up being the problem was that the skype database no longer knew how-to bill from paypal. Even while it had used that for another subscription the other day. There was also no bounced payment, it just forgot.
I knew when I read "cancelled" that it meant something else from earlier similar problems. Skype is quite messy and every time I get an email "Skype has improved XYZ" I do wonder what it ended up breaking. I know I should find an alternative, but as long as it sort of works...
These type of security issues do make me wonder again though.