GSM system outages...?

[Guest Samsonite]

I have had a trawl around the web to find some empirical data on a supposed GSM system issues that occurs on a regular basis and failed...

I have a customer who reports that the GSM modem my company sells is not reachable from midnight till 2am on a number of occasions. I know our equipment is good as it is reachable outside of these periods and nothing would have changed...

I have always thought that system updates occur at midnight or therabouts but the actual effect is minimal or invisible to the operation of the network capability.

Does anyone have any pointers or real data that could help me out here?

I know that a modem call is handled differently to a voice call but i am still unable to find any information. I spoke to the networks invloved with this and , bless the CS, i couldnt get any technical sense. No system downtime had been noted but my customer was reporting otherwise...

TIA