getting busy tone

Hi guys,

I am quite new to cisco callmanager and need help.
Its a strange scenario. There are two particular PSTN numbers that have tried to call our office all day and it keeps getting a busy tone. About 30 other numbers have tried and have called successfuly.
These two problem PSTN numbers have been calling our office in the past without any issues. The problem ony started today.

I have called the line provider and they have checked their end and found no problem at their end. All fingers are pointing to our phone system.

Please help as I don't know what to do here.