I am not sure if you are joking or not. Your logic seems sound and my own personal experience seems in agreement. Do you have a regular newsletter?

Then that is likely to be a clause in your contract of employment, that an amount specified is paid to you for making calls on your own phone or similar. It would be unreasonable for an employer to expect you to use your own personal phone to make calls on behalf of the business without reimbursement. Would you expect an employer to tell you to print off reams of company documents at home on your own printer using your own paper on a regular basis? Without reimbursement? Or drive 500 miles in your private car on company business without mileage allowance? The same rules (should) apply for mobile phones.