Has anyone successfully downgraded from the Unlimited data plan to the 200 MB one?

It would lower my bill by $15/mo, as the unlimited plan is $30/mo and the 200 MB one is $15/mo. That’s a $180/year difference.

But the AT&T website says the feature conflicts with “Basic Voice Mail”:

The following feature(s) are in conflict with an existing feature:
DataPlus 200 MB for iPhone
To add the new feature(s) you selected, the following feature(s) will need to be removed:
Basic Voice Mail

I’m afraid to continue because it sounds like voicemail will be removed. Is this true, or is the website wrong?