[hkr]

UMA is an interesting techology, but I have not been able to try it. Can someone please advise a link to a list of providers who support UMA?

However, I believe in protocols that are open to anyone: SIP allows anyone to start their own service, interface whatever with whatever... This helps keeping end-user prices down as there will always be fierce competition, whereas with UMA you are bound to your own mobile provider.

What I see as a major problem with wifi SIP roaming is that free wifi hotspots are not that easy to find, and even if you find them, they might not work properly with SIP. They allow web browsing, some allow POP3 or IMAP mail retrieval. Some SIP providers therefore allow connection using TCP rather than UDP, and on ports like 80 (normally used by http)... Problem is sometimes not firewals, but multiple NAT-ting and other problems. I have never tried to use SIP via VPN, but I have heard people say that it can often help.

However, I see that many people now use Skype and other VoIP software on their laptops for making calls while abroad. Using their phones for this is not yet that frequent. It is partly because a relatively small portion of phones available today offer wifi and voip, and those that do are relatively expensive.