How to make multiple calls without flash or on hook?

I have creased a DISA based on caller id will give the caller internal dialtone and then allow them to make calls out.

How can I make multiple phone calls without hanging up?

On calling card systems you normally wait for the other end to hang up, get dialtone or you press ## ?

How can I add such a feature into the system if it is not there?

Thanks

KM