MainActivity.cs

using Android.App;
using Android.Content.PM;

namespace MauiApp3;
[Activity(Theme = "@style/Maui.SplashTheme", MainLauncher = true, LaunchMode = LaunchMode.SingleTop, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation | ConfigChanges.UiMode | ConfigChanges.ScreenLayout | ConfigChanges.SmallestScreenSize | ConfigChanges.Density)]
public class MainActivity : MauiAppCompatActivity
{
    private NetworkStrengthWatcher? networkStrengthWatcher;

    protected override void OnStart()
    {
        base.OnStart();

        this.networkStrengthWatcher ??= new NetworkStrengthWatcher(this);
        // サクッとスタート
        this.networkStrengthWatcher.Listen();
    }

    protected override void OnDestroy()
    {
        // ここで止める
        this.networkStrengthWatcher?.Stop();

        base.OnDestroy();
    }
}

NetworkStrengthWatcher.cs

using Android.Content;
using Android.Telephony;

namespace MauiApp3;

public class NetworkStrengthWatcher
{
    /// <summary>
    /// 携帯ネットワーク通信に関するコールバックをまるめるインターフェース
    /// </summary>
    private interface ITelephonyState
    {
        /// <summary>
        /// 電波強度
        /// </summary>
        int Level { get; }
    }

    /// <summary>
    /// Android API Level 31 以降用
    /// </summary>
    private class TelephonyState : 
        TelephonyCallback, 
        TelephonyCallback.ISignalStrengthsListener, 
        ITelephonyState
    {
        public int Level { get; private set; }

        public void OnSignalStrengthsChanged(SignalStrength signalStrength)
        {
            // 電波強度を見やすいように 0 から 4 にしてくれているプロパティを使う
            this.Level = signalStrength.Level;

            Console.WriteLine($"Level: {this.Level}");
        }
    }

    /// <summary>
    /// Android API Level 30 以前用
    /// </summary>
    private class TelephonyStateOlder : PhoneStateListener, ITelephonyState
    {
        public int Level { get; private set; }

        public override void OnSignalStrengthsChanged(SignalStrength? signalStrength)
        {
            if (OperatingSystem.IsAndroidVersionAtLeast(31))
            {
                return;
            }

            base.OnSignalStrengthsChanged(signalStrength);

            if (signalStrength is null)
            {
                return;
            }

            // 電波強度を見やすいように 0 から 4 にしてくれているプロパティを使う
            this.Level = signalStrength.Level;

            Console.WriteLine($"Level: {this.Level}");
        }
    }

    private MainActivity activity;
    private TelephonyManager? telephonyManager;
    private ITelephonyState telephonyState;

    public NetworkStrengthWatcher(MainActivity mainActivity)
    {
        this.activity = mainActivity;

        this.telephonyManager = mainActivity.GetSystemService(Context.TelephonyService) as TelephonyManager;

        if (OperatingSystem.IsAndroidVersionAtLeast(31))
        {
            this.telephonyState = new TelephonyState();
        }
        else
        {
            this.telephonyState = new TelephonyStateOlder();
        }
    }

    public void Listen()
    {
        if (OperatingSystem.IsAndroidVersionAtLeast(31))
        {
            // 本当は MainExecutor プロパティは nullable なので見た方がいいかも。ここでは知ったこっちゃない扱いにしておきます。
            this.telephonyManager?.RegisterTelephonyCallback(this.activity.MainExecutor!, (TelephonyState)this.telephonyState);
        }
        else
        {
            this.telephonyManager?.Listen((TelephonyStateOlder)this.telephonyState, PhoneStateListenerFlags.SignalStrengths);
        }
    }

    public void Stop()
    {
        if (OperatingSystem.IsAndroidVersionAtLeast(31))
        {
            // 素直なメソッドがあるのでそれを使えば止まる
            this.telephonyManager?.UnregisterTelephonyCallback((TelephonyState)this.telephonyState);
        }
        else
        {
            // None を入れてリッスンすると止まる
            this.telephonyManager?.Listen((TelephonyStateOlder)this.telephonyState, PhoneStateListenerFlags.None);
        }
    }
}