Using a Smartphone for filmmaking means you are far more mobile than a conventional film crew. However when it comes to recording your sound you can limit your mobility with wired microphones. These connect to your Smartphone by cables that can constrain your interview subjects as well as restrict your movements.
The solution to this problem is to use a wireless microphone that works with a transmitter and a receiver. You would think there would be many different wireless microphones available but in truth only a few are any good. Until recently you have more choices: The BY-WM6 system features a bodypack transmitter, a camera-mount receiver, and an omnidirectional lavalier microphone. The receiver is built with mic and headphone output jacks. The bodypack transmitter includes a MIC jack and LINE IN jack.wireless microphone. Both are excellent wireless microphones.
You will also need to add an adaptor like a Rode SC4 to convert the TRS plug to a TRRS plug. Alternatively you may need a lightning or Type C 3.5mm adaptor so they can work with your Smartphone.
The BOYA BY-WXLR8 is a UHF wireless XLR transmitter designed to work with BY-WM6R and BY-WM8R.BOYA BY-WXLR8 UHF wirelessXLR Audio Transmitter with LCD Display for BY-WM8 BY-WM6 Wireless Lavalier Microphone System.
Now you can get just that with the surprisingly affordable, pro-level boya Wireless Lavalier Microphone.