I called back number (07) 5564 0567 rang once then goes to busy tone.

I too had this happen, it sounds very very dodgy. Beware.

The best case scenario puts callers from this number as a nuisance. When I answer it beeps constantly (like it does when being called by a fax machine). Could also potentially be a scam, like the "can you hear me?" scam. Have not yet spoken to anyone from this number, received two calls so far - which makes me suspicious.