[networkers]

3CX can certainly do all of the above.
3CX "takes over" the SIP accounts, so your phone registers with the 3CX server, and the 3CX server registers with your SIP accounts. In 3CX you can forward calls based on a number of parameters, time, callerID, SIP account and so on.
You will be able to route calls to either your phone, your wifes phone, ýour mobile phone, voice-mail... there's almost no limits.

You should try 3CX out, remember that there is a free version avaliable for you to try. (And keep :-) )

[leejor]

So how could we both see incoming calls for the same sip account?

With only a couple of users (extensions) and 7 trunks (providers). You would probably want to create a ring group with both extensions (or even more). This way all the phones would ring during an incoming call. If you had two appearances of the same extension number on each set, then you could use the hold feature on the set to take a second call.

This week my wife will get a SPA509G, and we would like to be able to forward calls from/to each other, and to talk as intercom. However, I cannot see how, as only one phone can register to a given sip account. So how could we both see incoming calls for the same sip account?

You only register 3CX to your providers, the sets then register to 3CX.

To complicate things even further we sometimes work in another country, and here we would like to just put the phones in our bag, take the plane, plug-in to the router in office-2, and bingo everything works like at home.

Many users do just this for travel (I do), or have extensions permanently, elsewhere in the world.

and not least you do not need a Linux box standing in a corner running 24x7x365

You will still need a Windows based PC running 24/7