I did one and they charged me a full month of service even though I returned the phone within the 30 return period, a week later due to weather. I returned it 5 days after purchase. They then proceeded to refuse to credit it back. There is nothing in the agreement that says you will be charged a minimum of one month service if you decided to cancel service and return the phone.
Perhaps it was my fault I didn't return the phone the next day. However the MSC did reimburse the activation fee, but it was too late to submit the monthly charges with the report because I only became aware of them a month after submitting the report.
Verizon then proceded to place the $75 debt on my credit report as an unpaid debt.