The case when contact capabilities are unknown is distinguished in getters and attributes of connection interface ContactCapabilities, but there is no way to signal when precise knowledge of contact's capabilities is lost, e.g. due to the contact going offline. Simon's plan to implement such a signal, quoted from #telepathy: <smcv> the backwards compat path would be to signal all possible caps for contacts whose real caps were unknown, then immediately signal ContactCapabilitiesUnknown to say "but actually that was just a guess"
(In reply to comment #0) > <smcv> the backwards compat path would be to signal all possible caps for > contacts whose real caps were unknown, In practice, and actually per definition of capabilities, it's important to signal caps that can be expected to work while nothing is known about contact's present capabilities. E.g. XMPP could signal text channel classes because offline messages normally work, but media channels need Jingle flavor data and therefore should be omitted. In Skype, contacts can have statuses like "offline but can leave voicemail", where a voice channel can be requested. There is also an issue of presence status lag, so it may be practical to announce voice for pure offline case too. With this consideration, the additional signal becomes more a matter of providing complete information, so I'm lowering the severity.
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