Get Discord: Discord for Android. Discord for iPhone. Most DMs and Players use this app/software/website for voice chat during D&D games.

Join The D&D Plaza Discord server. This can help you find online games to join. Tons of DMs. Tons of players. Been around a very long time. There are literally hundreds or thousands of D&D Discords, especially posted on Reddit, but this is the only one I recommend to everyone.

Highly Recommended (for Android only): Get CritDice. This will give you a really quick and easy way to roll dice just about anywhere, anytime. It can even announce your dice rolls, in case someone thinks you're cheating. Also includes potential bonus sound effects for rolling a 20 or a 1. (fun fun!)

As FarleyOcelot mentioned, DnDBeyond can give you a decent place to host/make your characters to play D&D on your phone. The site also comes with the minimal System Reference Document/SRD/"Basic Rules" so you can make simple characters. Then you have the option to buy tiny pieces of D&D content that aren't include for free for as little as $2 (for a class or a background, for example) or you can buy entire books or you can buy tiny pieces until you can afford an entire book, then the price of the entire book is discounted very nicely based on how many tiny pieces you already bought.