Holt-Winter forecasting for many future events


I have been investigating and testing forecasting methods for incoming call volumes.

I have many months of data.
The data exhibits a clear seasonality depending on the day of the week.

I have created a Holt-Winter model for the data with a period of 7, similar to the method shown in http://www.youtube.com/watch?v=VLlGO_fCdX0

My question is:
How do I extend the forecast for many future events (for example, 30 future events)
This method appears to be limited to forecasting only one additional cycle (in my case, 7 events).

I'm I misunderstanding the method, or is this expected for this type of forecast?
If expected, could you please provide advice on how to forecast t>7 for a 7 period data set?

Many thanks