diff --git a/src/applications/auth/controller/contact/PhabricatorAuthContactNumberPrimaryController.php b/src/applications/auth/controller/contact/PhabricatorAuthContactNumberPrimaryController.php
--- a/src/applications/auth/controller/contact/PhabricatorAuthContactNumberPrimaryController.php
+++ b/src/applications/auth/controller/contact/PhabricatorAuthContactNumberPrimaryController.php
@@ -55,7 +55,16 @@
         ->setContinueOnNoEffect(true)
         ->setContinueOnMissingFields(true);
 
-      $editor->applyTransactions($number, $xactions);
+      try {
+        $editor->applyTransactions($number, $xactions);
+      } catch (PhabricatorApplicationTransactionValidationException $ex) {
+        // This happens when you try to make a number into your primary
+        // number, but you have contact number MFA on your account.
+        return $this->newDialog()
+          ->setTitle(pht('Unable to Make Primary'))
+          ->setValidationException($ex)
+          ->addCancelButton($cancel_uri);
+      }
 
       return id(new AphrontRedirectResponse())->setURI($cancel_uri);
     }
diff --git a/src/applications/auth/factor/PhabricatorAuthFactor.php b/src/applications/auth/factor/PhabricatorAuthFactor.php
--- a/src/applications/auth/factor/PhabricatorAuthFactor.php
+++ b/src/applications/auth/factor/PhabricatorAuthFactor.php
@@ -62,6 +62,18 @@
     return null;
   }
 
+  /**
+   * Is this a factor which depends on the user's contact number?
+   *
+   * If a user has a "contact number" factor configured, they can not modify
+   * or switch their primary contact number.
+   *
+   * @return bool True if this factor should lock contact numbers.
+   */
+  public function isContactNumberFactor() {
+    return false;
+  }
+
   abstract public function getEnrollDescription(
     PhabricatorAuthFactorProvider $provider,
     PhabricatorUser $user);
diff --git a/src/applications/auth/factor/PhabricatorSMSAuthFactor.php b/src/applications/auth/factor/PhabricatorSMSAuthFactor.php
--- a/src/applications/auth/factor/PhabricatorSMSAuthFactor.php
+++ b/src/applications/auth/factor/PhabricatorSMSAuthFactor.php
@@ -28,6 +28,10 @@
     return 2000;
   }
 
+  public function isContactNumberFactor() {
+    return true;
+  }
+
   public function canCreateNewProvider() {
     return $this->isSMSMailerConfigured();
   }
diff --git a/src/applications/auth/xaction/PhabricatorAuthContactNumberNumberTransaction.php b/src/applications/auth/xaction/PhabricatorAuthContactNumberNumberTransaction.php
--- a/src/applications/auth/xaction/PhabricatorAuthContactNumberNumberTransaction.php
+++ b/src/applications/auth/xaction/PhabricatorAuthContactNumberNumberTransaction.php
@@ -83,6 +83,11 @@
         }
       }
 
+      $mfa_error = $this->newContactNumberMFAError($object, $xaction);
+      if ($mfa_error) {
+        $errors[] = $mfa_error;
+        continue;
+      }
     }
 
     return $errors;
diff --git a/src/applications/auth/xaction/PhabricatorAuthContactNumberPrimaryTransaction.php b/src/applications/auth/xaction/PhabricatorAuthContactNumberPrimaryTransaction.php
--- a/src/applications/auth/xaction/PhabricatorAuthContactNumberPrimaryTransaction.php
+++ b/src/applications/auth/xaction/PhabricatorAuthContactNumberPrimaryTransaction.php
@@ -41,6 +41,12 @@
           $xaction);
         continue;
       }
+
+      $mfa_error = $this->newContactNumberMFAError($object, $xaction);
+      if ($mfa_error) {
+        $errors[] = $mfa_error;
+        continue;
+      }
     }
 
     return $errors;
diff --git a/src/applications/auth/xaction/PhabricatorAuthContactNumberStatusTransaction.php b/src/applications/auth/xaction/PhabricatorAuthContactNumberStatusTransaction.php
--- a/src/applications/auth/xaction/PhabricatorAuthContactNumberStatusTransaction.php
+++ b/src/applications/auth/xaction/PhabricatorAuthContactNumberStatusTransaction.php
@@ -46,6 +46,12 @@
         continue;
       }
 
+      $mfa_error = $this->newContactNumberMFAError($object, $xaction);
+      if ($mfa_error) {
+        $errors[] = $mfa_error;
+        continue;
+      }
+
       // NOTE: Enabling a contact number may cause us to collide with another
       // active contact number. However, there might also be a transaction in
       // this group that changes the number itself. Since we can't easily
diff --git a/src/applications/auth/xaction/PhabricatorAuthContactNumberTransactionType.php b/src/applications/auth/xaction/PhabricatorAuthContactNumberTransactionType.php
--- a/src/applications/auth/xaction/PhabricatorAuthContactNumberTransactionType.php
+++ b/src/applications/auth/xaction/PhabricatorAuthContactNumberTransactionType.php
@@ -1,4 +1,72 @@
 <?php
 
 abstract class PhabricatorAuthContactNumberTransactionType
-  extends PhabricatorModularTransactionType {}
+  extends PhabricatorModularTransactionType {
+
+  protected function newContactNumberMFAError($object, $xaction) {
+    // If a contact number is attached to a user and that user has SMS MFA
+    // configured, don't let the user modify their primary contact number or
+    // make another contact number into their primary number.
+
+    $primary_type =
+      PhabricatorAuthContactNumberPrimaryTransaction::TRANSACTIONTYPE;
+
+    if ($xaction->getTransactionType() === $primary_type) {
+      // We're trying to make a non-primary number into the primary number,
+      // so do MFA checks.
+      $is_primary = false;
+    } else if ($object->getIsPrimary()) {
+      // We're editing the primary number, so do MFA checks.
+      $is_primary = true;
+    } else {
+      // Editing a non-primary number and not making it primary, so this is
+      // fine.
+      return null;
+    }
+
+    $target_phid = $object->getObjectPHID();
+    $omnipotent = PhabricatorUser::getOmnipotentUser();
+
+    $user_configs = id(new PhabricatorAuthFactorConfigQuery())
+      ->setViewer($omnipotent)
+      ->withUserPHIDs(array($target_phid))
+      ->execute();
+
+    $problem_configs = array();
+    foreach ($user_configs as $config) {
+      $provider = $config->getFactorProvider();
+      $factor = $provider->getFactor();
+
+      if ($factor->isContactNumberFactor()) {
+        $problem_configs[] = $config;
+      }
+    }
+
+    if (!$problem_configs) {
+      return null;
+    }
+
+    $problem_config = head($problem_configs);
+
+    if ($is_primary) {
+      return $this->newInvalidError(
+        pht(
+          'You currently have multi-factor authentication ("%s") which '.
+          'depends on your primary contact number. You must remove this '.
+          'authentication factor before you can modify or disable your '.
+          'primary contact number.',
+          $problem_config->getFactorName()),
+        $xaction);
+    } else {
+      return $this->newInvalidError(
+        pht(
+          'You currently have multi-factor authentication ("%s") which '.
+          'depends on your primary contact number. You must remove this '.
+          'authentication factor before you can designate a new primary '.
+          'contact number.',
+          $problem_config->getFactorName()),
+        $xaction);
+    }
+  }
+
+}