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This. We know that NSA and GCHQ collected records on Merkel's personal and work phones. There's no way they're not doing the same for Russian officials in the US, who would not be protected by the limitations on surveillance of US citizens. In the case of Trump's team, they may not be allowed to blanket collect the data, but there are definitely no rules against them finding out who is behind a phone number that turns up as the other party in their foreign surveillance.
Of course, may be possible that he actually was tapped, and has been advised to drop it because the NSA can just lie in legal hearings with impunity as they've done before.