John Goerzen suggested to skip Skype and Google Talk, and go for SIP phones directly.

One of the FAQ questions he lists is "Is SIP here to stay?". Well, in Germany, every major DSL reseller does offer Internet Telephony over SIP (the guys selling your normal land line of course do not...)

Apparently they even give you for a few Euros a DSL-Wireless Router where you can connect your phone, and which will do a least-cost-routing between your landline and your DSL SIP connection. I.e. if your destination host is a registered SIP number, you can call him for free using your normal phone! Also they have peering agreements, so you can usually call the users of the other companies also for free.

So SIP definitely is the protocol the industry in germany uses (apart from e.g. Cisco using it for their in-company phone network I've read somewhere)

Skype on the other hand, is a closed thing. Sure, they do offer Skype-out, but thats about it. There is no free peering between Skype and other services. From what I remember, Skype-out is also 2-3 times as expensive as what the DSL providers charge you here for the same service. Oh, and you don't need to have a DSL line with them, e.g. Web.de and Freenet.de let you subscribe for free, and you'll even get a german phone number, and with freenet 100 free minutes into the german PSTN.

Basically, Skype is an instant messenger which can do voice links. Just like google talk...

Well, google talk is new, and on their web page they suggest they might add SIP support later on. Then you could use Google Talk with your DSL providers account do dial out. If google plays nice (and they still have a record of playing nice, e.g. by using Jabber as their protocol)

SIP, while being a widely adopted standard, has its downsides, too.

I've been playing around with SIP half a year ago. There is lots of software available, and most works just fine. Asterisk is a full-blown PBX, you can do conference rooms with it (can you do conferences in Skype by now?), do interactive menus, gateways to PSTN, H.323, AIX... you name it.

Interaction between the numberous SIP clients usually works fine, but can be troublesome occasionally. (If more than one company would do Skype software they would have the same problem, I guess...)

Where Skype certainly is better, is in getting through firewalls. Which isn't really a good thing: firewalls are there for a reason... Well, anyway. Skype uses an ugly hack for this: it will promote regular Skype users with a good network connection (read: at universities and companies) to so called Super-Nodes. These then relay calls for people behind firewalls. So if you have a good connection, do NOT use skype... (apparently some universities and companies have already banned skype for that reason)

SIP users need to have one or more servers in the internet to relay their calls for them, if they are both firewalled. Well, not always, sometimes techniques such as STUN can be used to pierce the firewall. Anyway, if you often are behind some firewall and cannot setup a SIP proxy, you need to use an SIP provider. E.g. the DSL providers given above, or Free World Dialup mentioned by John.

Anyway, do try SIP. I can't await to see GnomeMeeting with SIP support in Debian, until then you can use e.g. linphone or kphone.