ArduinoBLE | ArduinoBLE library for Arduino

I'm trying to send a telephone number and a code (I'm using de code to read it in Arduino and know if this number is for one contact or other, because I'm gonna save different telephone numbers) vía ble from a Flutter app to a Arduino device (device: BLE 33). I'm using the ArduinoBLE library and the flutter_blue library.

For sending the number I need 6 bytes: for example for the number 112233445 one byte is for 11, second byte is for 22, third byte is for 33, 4º byte 44 and 5º byte is 5. And the code f.e. 4, is in another byte.

In Arduino, the function that I'm using to read is settingsCharacteristic.readValue(buffer, size_of_buffer). And in Flutter is characteristic.write(List).

I would like to send "04112233445" -> 4 is the code and the rest is the number.

The problem is that in Arduino I only receive 4 bytes.

Flutter code:

I'm realizing a BLE MIDI controller for iPad using an Arduino Nano 33 BLE device. The following code is able to:

• make the device discoverable as a BLE MIDI peripheral
• establish a connection with a BLE MIDI central application

The connection is stable only with Android apps. Each iOS app (eg Garageband, AUM, etc.) closes the connection immediately (the led on the arduino board turns on and off in few seconds), but if the device constantly sends MIDI messages (look at the lines of code commented in the loop() function) the connection remains active forever; unfortunately the repeated sending of messages is not the purpose of the controller that I want to realize.

There are probably specific configurations of the BLE service or polling actions to be implemented to comply with the strict iOS standards, but I could not find any working solution or example for the Nano 33 BLE device that does not include sending notes in the loop() function.

I'm trying to add custom payload in the BLE advertising packet and receive it from Android Service. (Arduino --- {data} ---> Android) However, ArduinoBLE library seems does not support a custom payload. (https://www.arduino.cc/en/Reference/ArduinoBLE)

So, How can I send data through the advertising sequence? (Not connect directly)

I am able to connect a Raspberry Pi 4 an and Arduino Nano BLE through bluepy for Rpi4 and ArduinoBLE.h for Arduino Nano BLE. Unfortunately when I try to write from Rpi4 to Arduino Nano BLE, I'm not seeing the expected output for Read and Write. I don't see any perfect example for Arduino Nano BLE since it is recently released hardware with built-in BLE. It would be very helpful if anyone could help me achieve communication between them. Thanks in advance. Below is my code for Raspberry Pi.

I am using the ArduinoBLE library to create a service and characteristic:

I am currently using an Arduino Nano 33 BLE to calculate an angle measurement from an IMU and add these values to a Bluetooth characteristic within a service. I create the service and characteristic using the ArduinoBLE library: