make a phone call click on a button

Have you given the permission in the manifest file

 <uses-permission android:name="android.permission.CALL_PHONE"></uses-permission>   

and inside your activity

  Intent callIntent = new Intent(Intent.ACTION_CALL);
    callIntent.setData(Uri.parse("tel:123456789"));
    startActivity(callIntent);

Let me know if you find any issue.

There are two intents to call/start calling: ACTION_CALL and ACTION_DIAL.

ACTION_DIAL will only open the dialer with the number filled in, but allows the user to actually call or reject the call. ACTION_CALL will immediately call the number and requires an extra permission.

So make sure you have the permission

uses-permission android:name="android.permission.CALL_PHONE"

in your AndroidManifest.xml

<manifest
    xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.dbm.pkg"
    android:versionCode="1"
    android:versionName="1.0">

    <!-- NOTE! Your uses-permission must be outside the "application" tag
               but within the "manifest" tag. -->

    <uses-permission android:name="android.permission.CALL_PHONE" />

    <application
        android:icon="@drawable/icon"
        android:label="@string/app_name">

        <!-- Insert your other stuff here -->

    </application>

    <uses-sdk android:minSdkVersion="9" />
</manifest> 

change your String to String phno="tel:10digits"; and try again.

None of the above worked so with a bit of tinkering here's code that did for me

        Intent i = new Intent(Intent.ACTION_DIAL);
        String p = "tel:" + getString(R.string.phone_number);
        i.setData(Uri.parse(p));
        startActivity(i);