I wasn't asked to give alternatives though. And I intentionally didn't give any examples because sometimes you can end up sounding like a car salesman or employee of said company. Since you are now asking specifically though, I'll bite.
I use VoipStunt.com for sip calling. It's actually brilliant. A tenner of credit every 120 days and you get 120 days of free calls to about 30 countries. The tenner of credits is still good after that has expired. I've been using it since 2003. You used to be able to get London numbers through them, but you can pick them up for a couple of bucks a month from flynumber.com or just get an 056 number for free if you don't mind that prefix.
With SIP you only need 64kbps for a voice call, and there is no leeching of your bandwidth or CPU. I prefer a hardware device for calls so I have a phone that I can use even if my pc is shutdown. They have a crappy client, which works fine mind you, but you can use ANYTHING you want by just plugging in the settings (unlike Skype). We're even setting up a proper answering machine service on our server to handle calls for our company now.
For chat I use Pidgin.im - again there are plenty of alternatives and I have several accounts of several protocols running there (different clients use different protocols).
Skype is the horsemeat equivalent of a juicy aussie steak.