I'd soon change my number before resorting to most apps. Unless it's like uBlockOrigin where I just feed blacklists into it, I'm not really okay with giving an organization besides my service provider my call history. Read Nomorobo's TOS sometime, it's a doozy.
Of course, this means that call blocking apps have less features than on Android. For example, apps can't dynamically look up a number when a call is revived and make an on-the-fly decision. This is in keeping with iOS' philosophy of "privacy/security over features", vs. Android's "everything is completely open to developers, for better or worse".
http://www.nomorobo.com/pages/privacy
Some highlights from the legalese:
> 1.2 Data Privacy. You understand and agree that some of your call information (including, but not limited to, a log of all phone calls made to your subscribed phone line(s) and any requested additions to any customizable phone number blacklist or white list) may be viewable by you, the Company, and by any other person having a phone line subscribed to the Nomorobo Service through the same user account as you.
I don't know that that is actually possible on iOS, though, as iOS doesn't give the call log to the blacklist provider.
>1.3 Contact. From time to time, the Company may need to send e-mails, in-app messages, and/or push notifications to you and automated voice calls and/or text messages to all phone lines that are subscribed to or otherwise using the Nomorobo Service.
Could be innocuous now and be much more annoying later if they decide to change their business model.
>Binding Arbitration. If the parties to the Agreements do not reach a solution through the informal resolution process described in Paragraph 6.3(ii), then any controversy or claim arising out of or relating to the Agreements, shall be settled by arbitration administered by the American Arbitration Association (the "AAA") in accordance with its Commercial Arbitration Rules, and judgment on the award rendered by the arbitrator(s) may be entered in any court having jurisdiction thereof or having jurisdiction over the relevant party or its assets.
Must go to arbitration if anything goes wrong and you want to sue.
Overall, not the worst I've seen. There was another one that you had to grant permission to post to your Facebook wall and/or Twitter account. I don't recall which one it was, though.