I'm a big believer that the mobile carriers are ideally positioned to become VoIP players of significance.
They already have customers, billing platforms and put handheld devices in peoples hands. They have both direct and indirect sales agents, and roaming agreements that allow you to take your phone number with you.
So, without any real surprise, I saw that T-Mobile's sister company enter into a VoIP deal in Spain. I think this is just a tip of an iceberg, and T's total solution plan shedding some light. WiFi, Mobile, VoIP. Hmmm.