I believe it's just part of the soundtrack & as Molly said it is used to signify to the audience that his head is indeed 'ringing' from the knock.
I don't believe it is necessarily a phone ringing, lots of bells & buzzers sound similar; its just an instantly recognised sound " Oh that's ringing".
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Also, please note that sentences can also end in full stops. The exclamation mark can be overused.
Sherlock Holmes 28 March 13:08
Mycroft’s popularity doesn’t surprise me at all. He is, after all, incredibly beautiful, clever and well-dressed. And beautiful. Did I mention that?
--Mark Gatiss
"I know that you believe you understand what you think I said, but I’m not sure you realize that what you heard is not what I meant."
Robert McCloskey