[VoiceOps] Past due/overdue invoices for Doctor offices - How to handle?

Hi All, What's the general rule/experience when dealing with Doctor offices that have unpaid invoices on postpaid telephone service plans? This may vary largely in different regions/states, but can a VoIP provider threaten to disconnect telephone service for unpaid invoices? If a Doctor office is on a pre-paid plan, like most VoIP operations, are is there any liabilities on the VoIP provider when the balance falls below a 0 or negative balance and there happens to be a medical emergency? Perhaps it would be possible to disable calling to everyone except 911. Is this a legitimate practice? Thanks!