The "the Airtel number you are calling is currently switched off" is something that I have lived with for months at Wilson Airport!

I have a specific number (0733 325 666) which experiences this problem every now and then. This number is installed on a GSM Gateway with 4-SIM modules, together with other numbers from Airtel. However, this particular number has given me so much grief.
I have changed the SIM slot on which it is installed, I have even installed it on another GSM Gateway where I use only Safaricom lines - I have done everything. I have even swapped the SIM card for a new one, but that has not solved the problem.
I have reported the problem to Airtel and at some point they said something like the SIM card is being served from a BS in town (I am at Wilson Airport) so I don't understand this issue with BS in town, etc.

My problem is: Why would one number subject me to so much grief when all other numbers work fine?

Airtel (through the Account Manager assigned to us) asked us to be sending them reports (via e-mail) whenever this problem occurs. We have done that, but they have in turn done nothing! I'd have thought they'd assign an engineer from their IT section to look at this, but it seems their engineers are least bothered by such small problems.

What can I do, apart from porting this number to QB??