Here's an interesting link to Optimus Portugal's IMS offer for their customers, translated into English thanks to the help of Google. Looks like the offer is based on an IMS core of Ericsson and a IMS client of Movial. Since it is the first IMS offer I have seen being deployed in a real network for real customers I took a closer look to see exactly what is offered and how it works:
First, there is no software being deployed on mobile devices, they keep functioning as before. Instead, Optimus uses a PC based IMS client, so their subscribers can make and receive calls from both their mobile phone and their PC. So far, the offer takes the simplest approach: Calls are either received on the mobile device or on the PC, the user has to choose.
While the user is connected to the network with the IMS PC client, other users also online via the PC client can be called for free. This involves presence, i.e. the PC clients can see each other. Also, instant messaging and video calling are free between PC clients.
SMS is interesting, too. While the PC client is active, SMS message are routed to the client and not to the mobile phone. However, this only applies to SMS messages being sent by other Optimus users. SMS message coming from other networks are still delivered to the mobile device. That's probably got something to do with SMS message routing, as SMS Service Centers in other networks can deliver the messages directly to the mobile device without going to an Optimus SMSC first. A bit of a catch.
Another plus they are advertising is that calls from abroad via the PC client are charged at the standard rate as if you were in Portugal. Incoming calls to the PC client would be free, no matter how far you are away from your home country.
It looks like the IMS PC client and the mobile device are still strictly separated and all IMS parts of the system are only used for the PC client. A good way to start working with IMS, especially for mobile operators with fixed line (DSL) assets.
I wonder, however, if this is a bit of a dangerous path for a mobile operator to walk as some fixed line operators could start wondering why interconnection charges are higher even if a call is not delivered via the cellular network but via a fixed line PC client.
Would I be compelled to use the service? Once I am at home I wonder if for free voice and video calls, Skype with its superior voice and video quality would be better for me. SMS might be more convenient to use on the PC as typing is easier but I would still have to keep an eye on the mobile device in order not to miss incoming messages from people using other networks. And for instant messaging, I have a program already in place as well.
So I guess I would try it, with the Optimus IMS client being one more application running in the background, which I hopefully do not forget to switch off before I leave the house or I would not receive incoming calls. A good place to start from, I hope they keep expanding the functionality.