in many forums, apple iphone customers facing issues to reconnect when they accidentally hit forget device. I faced the same issue and contacted apple support. I've was advised to reset network settings but it didn't helped.
Many of the third party bluetooth headsets are not coming in market with 3 buttons - one volume up/down and a power key.
When u hit forget device on yur iphone and the bluetooth headset is not showing up again,
do
1. turn on bluetooth on yur iphone
2. press & hold power on button on yur bluetooth headset
3. press and hold power on and volume down key till you hear 2 beep sounds.
4. Now the device will show up in yur iphone