We were down from about 10am to 4pm EST (UTC -5). We use the linkpoint API, we do have a failure notification system indicating the failure. Doing something about that failure when your phones start going crazy and you start seeing problems can be difficult however. This all depends on volume, a seller with few transactions may never see the problem, if you have a large number of transactions you will find out the problem very quickly regardless of your notification method.
We were able to take some action to temporarily just record the cc numbers until we could process them later so this was transparent to the customer but we had lots of transactions that required same day shipment and we could not verify payment until late in the day so this put us in a serious bind.
My link was removed but if you go to linkpoint's primary website and go to the gateway status you will a stoplight graphic that shows network status. As soon as we found our processing down (which was nearly immediate), we checked the gateway status and their own stoplight graphic showed that their network was down and was not restored until about 4pm the same day.
We intend to change our code such that the cc processing system on our side performs a gateway status check. If it fails, it will automatically revert to cc# storage mode until the gateway comes back up again so that we do not risk losing sales as we determine that the gateway is down (via our notification system) and switch over manually to a cc# recording mode). An alternative is not to perform real-time validation of the card but we prefer to do this to prevent the headache of calling or notifying thousands of customers about zip code, expiration date, adress, etc problems.
Glad to hear your service stayed up but ours was down and their own site indicated that they were down. In our time with linkpoint, which has been about 18 months they have gone down about a dozen times and their status has indicated such (they also send an email later about the problem if you turn on their diagnostic messages). This is the only time we have been down for the majority of a business day.