Intent intent = new Intent(Intent.ACTION_CALL);
<uses-permission android:name="android.permission.CALL_PHONE" />
At least in the US, 911 is an emergency number.
CALL_PHONE is insufficient to call that number. There is a separate permission for that (
CALL_PRIVILEGED?), one that cannot be held by ordinary SDK apps.
UPDATE: I remembered correctly. Here is the Android 6.0 platform manifest entry for that permission:
<!-- @SystemApi Allows an application to call any phone number, including emergency numbers, without going through the Dialer user interface for the user to confirm the call being placed. <p>Not for use by third-party applications. --> <permission android:name="android.permission.CALL_PRIVILEGED" android:protectionLevel="signature|privileged" />