The Open Mobile Terminal Platform , a mobile-phone industry body which counts Orange, Vodafone, T-Mobile and 3 amongst its membership, has published guidance for network operators and handset manufacturers on provisioning and maintaining VoIP settings on new handsets.
The document only covers VoIP clients pre-installed on handsets, such as that used by Truphone, as opposed to applications which are downloaded later, such as Fring or Vyke.
According to the OMTP specifications, operators are entitled to remove or lock down VoIP applications on subsidised handsets, but they must provide the ability to remove that lock when the contract period expires, just as they now will release a handset to be used on another network (the SIM lock).
It must also be clear to users that the capability has been locked. So the removal of menu items, in the way that Vodafone and Orange crippled their N95 variants, wouldn't be allowed. Users selecting a locked option should he informed that the operator has disabled the function.
The document specifies around 15 functional requirements, though members aren't bound to conform to all, or indeed any, of them. But as so many network operators were involved in writing the thing, it's hoped they'll pay more than a passing interest.
The specification is available, in its entirety, from the OMTP site.