Remember that they need to prove the complaint, not you defend.
So far you can demonstrate that you have not made a call so down to someone “maybe” calling you.
No photographic evidence of alleged offence and the person complaining was so serious about it that they didn’t even give their name.
You need balance of probability to dismiss someone and I can’t see how they have that. It is your word again “unknown” person with no evidence. You can show you had not made a call so it down to them to show why they think someone called you.
I certainly wouldn’t dismiss you on what you have said, wouldn’t even take any action. If they really are crazy enough to sack you get your appeal in and then take to the tribunal. I think you said you work for a large company? I am willing to bet that things change very quickly when it gets to the legal department at head office…