The best alternatives to Castle are Dasha, CallHero, and Scam Spotter by Google. If these 3 options don't work for you, we've listed a few more alternatives below.
What do you think of Castle?
AssemblyAI— Speech-to-Text API with superhuman accuracy
Dasha is the first and only AI platform that allows you to design, test and launch rich, human-level voice conversations. Automate your entire work flow and bring your costs down or sell your own solutions built on top of Dasha.
Callhero screens incoming calls for you. Not sure about a call? decline to forward it to Callhero. You’ll immediately get a text notification with who’s calling and why. You can then answer, mark as spam or request more info.
RoboStopper is a cross-platform app that blocks all unwanted phone calls, with an optional bypass rate that spammers must pay. No foolhardy blacklists, and more flexibility than iOS’s Silence Unknown Callers so that you don’t miss any important phone calls.
MyHumans blocks 100% of spam calls. Stop wasting your time trying to block scammers that can fake or recycle phone numbers. Receive just the calls you want using a whitelist of your contacts and send the rest to voicemail without disturbing you.
Callthentic prevents spam calls to your iPhone. It compliments the new iOS 13 "Silence Unknown Calls" feature. Callers not in your phone’s contact list are challenged for the call to complete. No junky voicemail to deal with either. $1.99 monthly sub price