A few years ago (2010-11) the IRS changed its treatment of cell phones, basically making them a de minimis fringe benefit not subject to taxation. Provided the reason your organization pays for the phone is because the employee must be available during non-standard times, then the IRS does not require you to keep a log of business and personal use.
Your organization is free to reimburse employees based on the criteria you set. If it is your policy to only reimburse for business use, then you’d want a detailed breakdown in an expense report. If not, then a more general summary listing the cost per phone would be sufficient. It depends on your own policies.
We do not have benchmarks on company cell phone policy; anecdotally, it appears that where a company provides a phone or reimbursement for a phone, they cover/reimburse the entire cost rather than enforce/reimburse “business use” only, which is not cost-effective and not required by the IRS. But we do not have figures on how many companies pay for cell phones for employees.