You apparently ignored all of the information provided during the setup process. The app specifically states that in order for it to function, you must allow it to intercept all calls sent to your regular voicemail. It also points out the fact that it CAN NOT automatically block calls. (The iOS operating system does not permit developer access to such functions.)
What WhoApp does do, is provide you with caller ID information for numbers coming in, without full identification. This allows you to ignore telemarketing, spam and other unwanted calls. If you want to completely block a number; you have to do that via the iPhone's phone app. Go to "recent calls" and locate the number you want blocked. Select the circled "i" button, to the right of that number. This will bring up a screen; where you can choose to have that number blocked.
Regarding your voicemail and uninstalling the app; WhoApp informs you that it is creating a new voicemail system (since, in order to function, it must intercept calls destined for your carrier's/apple voicemail, in order to identify the caller.) The final step of installation, sends you to a web page; which provides you with full uninstall instructions. Those instructions include the following:
To deativate WhoApp, please find your carrier and follow these instructions (include all special characters * and #):
AT&T: call this number, ##004#: If that does not work call ##61#, then call ##62#, then call ##67#
T-Mobile: call this number, ##004#
Metro-PCS: call this number, ##004#
Sprint: call *730, then call *740
Verizon: call *73: if that does not work call *920, then call *900
US Cellular: call *920 then call *900
WhoApp was created by a developer who won a $25,000 competition, sponsored by the FCC. The goal was to create the best application to assist individuals in avoiding annoying Spam / telemarketing calls. While the FCC has had its "Do not call" registration list for several years; scammers and spammers will simply ignore that list. This is one of the few options available to iPhone users to avoid such nuisance calls.