Hi and thanks. One of your collegue has made some suggestions on another part of the forum: https://supportforums.cisco.com/docs/DOC-9889#cf and meanwhile, support EMEA has escalated the case as well. I was so happy the issue was picked up that I forgot that I had to re-state my question on this part of the forum, so sorry for any confusion.
the problem was solved: the ACL was too generous and allowed external (malicious) servers to probe the phones, which confused the phones. I found out only after examining a copy of the traffic to and from a phone.
Unfortunately, the 'restricted domains' paramater did not prevent the phones from being confused.
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