diff --git a/lib/sms.php b/lib/sms.php
index 13e5691ace..669b0c1242 100644
--- a/lib/sms.php
+++ b/lib/sms.php
@@ -12,10 +12,11 @@
 /**
  * Send an SMS message to the specified numbers using the Twilio API
  *
- * @param int|array Nnumber(s) to send an SMS to. Accepts formatted and unformatted US numbers, e.g. +14155551212, (415) 555-1212 or 415-555-1212.
+ * @param int|array Number(s) to send an SMS to. Accepts phone numbers in E.164 format e.g. +14155551212
  * @param string Message to send. Longer than 140 chars will be split.
  *
  * @link http://www.twilio.com/docs/api/rest/sending-sms
+ * @link https://www.twilio.com/docs/glossary/what-e164
  */
 function send_sms( $to, $message ) {
 
diff --git a/wpcom-vip-two-factor/sms-provider.php b/wpcom-vip-two-factor/sms-provider.php
index ca641eb6d3..b1da542865 100644
--- a/wpcom-vip-two-factor/sms-provider.php
+++ b/wpcom-vip-two-factor/sms-provider.php
@@ -201,12 +201,15 @@ public function user_options( $user ) {
 			<?php else : ?>
 				<label>Phone Number
 					<input name="vip-two-factor-phone" type="tel"
+						pattern="^\+[1-9]\d{1,14}$"
 						value="<?php echo esc_attr( $sms ); ?>"/>
 				</label>
+				<!-- Requires E.164 format for telephone numbers https://www.twilio.com/docs/glossary/what-e164 -->
 				<input type="submit" class="button" name="vip-two-factor-phone-send-code"
 					value="<?php esc_attr_e( 'Submit', 'two-factor' ); ?>"/>
-				<p><strong>Note:</strong> Please include your country calling code (e.g. +44, +1, +61, etc.) to ensure
-					SMS messages are correctly sent.</p>
+				<p><strong>Note:</strong> Please include your country calling code (e.g. +44, +1, +61, etc.) so that the
+					phone number is as per <a href="https://www.twilio.com/docs/glossary/what-e164" target="_blank" rel="noopener">E.164 standards</a>,
+					thereby ensuring that the SMS messages are sent correctly.</p>
 			<?php endif; ?>
 		</div>
 		<?php