I get an email every time someone posts on a thread that I've posted on. But I don't have my phone set up to notify me of emails from that account. If I did, then I'd get a phone notification of a new email, which would then say, "New Reply to -thread- at Polyamory.com" and I'd know...
If such a feature exists, it'd probably be in the "User CP" (control panel) which is the first link in the little blue strip of linkies near the top of the screen...that's where all the stuff to twiddle with your settings is located.
The default is to be notified of any new posts in threads in which you've participated. Once you've visited the thread and read the new posts, you won't be notified again until someone contributes something new. You can opt out of this notification in your User CP.
You can also subscribe to threads in which you haven't posted by clicking on the Thread Tools link at the top of each thread and selecting "Subscribe to this Thread." You can see all your subscriptions in User CP.
I'm not sure, but I think these instructions might be in the user guidelines, somewhere.
The User CP (User Control Panel) has a column along the left-hand side for viewing and tweaking various features. Under "Settings & Options" find "Edit Options" ... and click on that. Then you'll see a large box with various things you can change. Under "Messaging & Notification" find "Default Thread Subscription Mode" and a drop-down menu. From there you can select various options, including "Instant email notification" ... select the one you want, then scroll to the bottom of the page and click on the "Save Changes" button. After that, you just have to check your email, and you will receive notifications.