Many services have a webpage you can use to add/manage account credit. It's not possible to do this per-protocol, in the client, because of services like SIP and XMPP who can have arbitrary PSTN gateways so instead it should be a property given by the CM.
http://git.collabora.co.uk/gitweb/?p=user/danni/telepathy-spec.git;a=shortlog;h=refs/heads/manage-credit-36254
Looks fine. I added this: This property is not guaranteed to be well-defined until the connection becomes Connected; there is no change notification. I didn't add the immutable flag. Merged for 0.22.2
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