IPhone disconnect after one ring

Normally, you can choose for the phone to ring between 5 and 30 seconds before the call is forwarded to Voice Mail or another phone number.

However, this is a Carrier feature, so you must contact your Carrier and ask them to change the time before calls are forwarded. You can also change it directly in the telephone keyboard on your iPhone, but this requires a specific code, which can be different from carrier to carrier, so you can search for information on your carrier's web page.