You can write the message as follows;
The Manager,
Company Name,
Address.
Subject: Wrong Bill Issued
Respected Sir/ Madam,
With due reverence, it is stated that I have been charged some phone calls which I never made in my last bill. It might be due to some technical fault from your side. It has never happened before and I request you to check my bill again and correct it accordingly.
Yours truly,
XYZ
The Manager,
Company Name,
Address.
Subject: Wrong Bill Issued
Respected Sir/ Madam,
With due reverence, it is stated that I have been charged some phone calls which I never made in my last bill. It might be due to some technical fault from your side. It has never happened before and I request you to check my bill again and correct it accordingly.
Yours truly,
XYZ