An owner of the tp-link TD-W9960v (not offering the possibility to set a proxy for the SIP connection), who has O2 as provider and is experiencing the same issues, reports the following (impractical) workaround: When outgoing calls fail, he logs in and checks or unchecks the box for IPv6 under network. This forces a short reconnection, upon which outgoing calls work (for a while).

This makes it seem like the connection can be failing while reporting success. Could it be a timeout issue?

With the proxies redundantly set to the provider's SIP-server, it has been running for over a week without problems now, including the weekend. Unless O2 fixed something with their servers, this seems to help therefore. :-)