Do you mean email notifications? In your forum control panel:
http://www.abandonia.com/vbullet/pro...do=editoptions
Scroll down to "Default Thread Subscription Mode"
And per individual thread:
http://www.abandonia.com/vbullet/sub...ewsubscription
Even in the past century when our decaying forum board was developed, we had such options.
No iPhone app though.