Thanks for your post!
Unlike traditional cell phone providers, there's no number to call to get your voicemail messages; rather, you access them like any other message or call log, right from your app or online at www.textnow.com.
The voicemail files themselves are very small, so you would have to have literally thousands of undeleted voicemails before you started running out of space.
You delete them like any conversation - swipe on it on the conversation page and tap on the garbage-pail icon.
Hope that helps!