Your company president not only missed the boat on this one, he missed the entire harbor. If you were constantly failing to return phone calls, and he had given you feedback and a clear idea of the consequences if this problem continues, then some disciplinary action could be taken. But, in your case, you didn't even fail to return the calls in the first place.
The real failure here was that he did not listen to you. He simply let the tape roll and assumed you are guilty. And further, his strategy of threatening employees when he thinks they have made mistakes totally misses a key function of management, namely treating employees with respect and trust.
This situation called for a discussion of what happened and what to do to prevent this kind of problem in the future. Two-way communication is essential for a telephone to work, and it is essential for a manager to work, too.
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