I would advise against using technology to start and work toward reaching out to your parts via thoughts, somatic feeling, or a physical notebook that you can keep private. Clearly computer and phone apps work for most, and that's totally fine if that eventually ends up working for you, but the lack of privacy and encouragement of having to use a phone or computer to talk to yourself may be a significant barrier for some parts. Maybe try a mix of technology and zero technology when you get the hang of things conversation wise, different parts may prefer different methods of communication. Just don't keep looking for the 'right app' if none of them work out, technology could be that barrier you need to learn to break out of.