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

[jungle Boogie]

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!