The following is from Cisco 5.x SRND about the overlap sending and receiving
Overlap Sending and Overlap Receiving
In countries whose national numbering plan is not easily defined with static route patterns, you can configure Cisco Unified CallManager for overlap sending and overlap receiving.
Overlap sending means that Cisco Unified CallManager keeps collecting digits as they are dialed by the end users, and passes them on to the PSTN as they are dialed. To enable overlap sending in Cisco Unified CallManager Release 4.0 or later, check the Allow Overlap Sending box on the Route Pattern Configuration page. (In earlier Cisco Unified CallManager releases, overlap sending is enabled by setting the SendingCompleteIndicator service parameter to False.) The route pattern needs only to include the PSTN access code (for example, "9." in North America or "0." in many European countries).
I still not clear even after reading this, can anybody give me an example how this is achieved. I haven't seen this useful at least in U.S. Thanks