diff --git a/resources/sql/autopatches/20160218.callsigns.1.sql b/resources/sql/autopatches/20160218.callsigns.1.sql
new file mode 100644
--- /dev/null
+++ b/resources/sql/autopatches/20160218.callsigns.1.sql
@@ -0,0 +1,4 @@
+/* Make callsigns nullable, and thus optional. */
+
+ALTER TABLE {$NAMESPACE}_repository.repository
+  CHANGE callsign callsign VARCHAR(32) COLLATE {$COLLATE_SORT};
diff --git a/src/applications/diffusion/controller/DiffusionRepositoryCreateController.php b/src/applications/diffusion/controller/DiffusionRepositoryCreateController.php
--- a/src/applications/diffusion/controller/DiffusionRepositoryCreateController.php
+++ b/src/applications/diffusion/controller/DiffusionRepositoryCreateController.php
@@ -149,14 +149,6 @@
 
         // If we're creating a new repository, set all this core stuff.
         if ($is_create) {
-          $callsign = $form->getPage('name')
-            ->getControl('callsign')->getValue();
-
-          // We must set this to a unique value to save the repository
-          // initially, and it's immutable, so we don't bother using
-          // transactions to apply this change.
-          $repository->setCallsign($callsign);
-
           $xactions[] = id(clone $template)
             ->setTransactionType($type_name)
             ->setNewValue(
@@ -343,7 +335,7 @@
   }
 
 
-/* -(  Page: Name and Callsign  )-------------------------------------------- */
+/* -(  Page: Name  )--------------------------------------------------------- */
 
 
   private function buildNamePage() {
@@ -359,23 +351,7 @@
       ->addControl(
         id(new AphrontFormTextControl())
           ->setName('name')
-          ->setLabel(pht('Name'))
-          ->setCaption(pht('Human-readable repository name.')))
-      ->addRemarkupInstructions(
-        pht(
-          '**Choose a "Callsign" for the repository.** This is a short, '.
-          'unique string which identifies commits elsewhere in Phabricator. '.
-          'For example, you might use `M` for your mobile app repository '.
-          'and `B` for your backend repository.'.
-          "\n\n".
-          '**Callsigns must be UPPERCASE**, and can not be edited after the '.
-          'repository is created. Generally, you should choose short '.
-          'callsigns.'))
-      ->addControl(
-        id(new AphrontFormTextControl())
-          ->setName('callsign')
-          ->setLabel(pht('Callsign'))
-          ->setCaption(pht('Short UPPERCASE identifier.')));
+          ->setLabel(pht('Name')));
   }
 
   public function validateNamePage(PHUIFormPageView $page) {
@@ -387,38 +363,7 @@
         pht('You must choose a name for this repository.'));
     }
 
-    $c_call = $page->getControl('callsign');
-    $v_call = $c_call->getValue();
-    if (!strlen($v_call)) {
-      $c_call->setError(pht('Required'));
-      $page->addPageError(
-        pht('You must choose a callsign for this repository.'));
-    } else if (!preg_match('/^[A-Z]+\z/', $v_call)) {
-      $c_call->setError(pht('Invalid'));
-      $page->addPageError(
-        pht('The callsign must contain only UPPERCASE letters.'));
-    } else {
-      $exists = false;
-      try {
-        $repo = id(new PhabricatorRepositoryQuery())
-          ->setViewer($this->getRequest()->getUser())
-          ->withCallsigns(array($v_call))
-          ->executeOne();
-        $exists = (bool)$repo;
-      } catch (PhabricatorPolicyException $ex) {
-        $exists = true;
-      }
-      if ($exists) {
-        $c_call->setError(pht('Not Unique'));
-        $page->addPageError(
-          pht(
-            'Another repository already uses that callsign. You must choose '.
-            'a unique callsign.'));
-      }
-    }
-
-    return $c_name->isValid() &&
-           $c_call->isValid();
+    return $c_name->isValid();
   }
 
 
diff --git a/src/applications/diffusion/controller/DiffusionRepositoryEditMainController.php b/src/applications/diffusion/controller/DiffusionRepositoryEditMainController.php
--- a/src/applications/diffusion/controller/DiffusionRepositoryEditMainController.php
+++ b/src/applications/diffusion/controller/DiffusionRepositoryEditMainController.php
@@ -284,7 +284,12 @@
       $repository->getVersionControlSystem());
 
     $view->addProperty(pht('Type'), $type);
-    $view->addProperty(pht('Callsign'), $repository->getCallsign());
+
+    $callsign = $repository->getCallsign();
+    if (!strlen($callsign)) {
+      $callsign = phutil_tag('em', array(), pht('No Callsign'));
+    }
+    $view->addProperty(pht('Callsign'), $callsign);
 
     $short_name = $repository->getRepositorySlug();
     if ($short_name === null) {
diff --git a/src/applications/repository/storage/PhabricatorRepository.php b/src/applications/repository/storage/PhabricatorRepository.php
--- a/src/applications/repository/storage/PhabricatorRepository.php
+++ b/src/applications/repository/storage/PhabricatorRepository.php
@@ -93,7 +93,7 @@
       ),
       self::CONFIG_COLUMN_SCHEMA => array(
         'name' => 'sort255',
-        'callsign' => 'sort32',
+        'callsign' => 'sort32?',
         'repositorySlug' => 'sort64?',
         'versionControlSystem' => 'text32',
         'uuid' => 'text64?',
@@ -149,13 +149,21 @@
   }
 
   public function getMonogram() {
-    return 'r'.$this->getCallsign();
+    $callsign = $this->getCallsign();
+    if (strlen($callsign)) {
+      return "r{$callsign}";
+    }
+
+    $id = $this->getID();
+    return "R{$id}";
   }
 
   public function getDisplayName() {
-    // TODO: This is intended to produce a human-readable name that is not
-    // necessarily a global, unique identifier. Eventually, it may just return
-    // a string like "skynet" instead of "rSKYNET".
+    $slug = $this->getRepositorySlug();
+    if (strlen($slug)) {
+      return $slug;
+    }
+
     return $this->getMonogram();
   }
 
@@ -699,7 +707,13 @@
   }
 
   public function getURI() {
-    return '/diffusion/'.$this->getCallsign().'/';
+    $callsign = $this->getCallsign();
+    if (strlen($callsign)) {
+      return "/diffusion/{$callsign}/";
+    }
+
+    $id = $this->getID();
+    return "/diffusion/{$id}/";
   }
 
   public function getPathURI($path) {
@@ -708,7 +722,12 @@
 
   public function getCommitURI($identifier) {
     $callsign = $this->getCallsign();
-    return "/r{$callsign}{$identifier}";
+    if (strlen($callsign)) {
+      return "/r{$callsign}{$identifier}";
+    }
+
+    $id = $this->getID();
+    return "/R{$id}:{$identifier}";
   }
 
   public static function parseRepositoryServicePath($request_path) {
@@ -1063,7 +1082,13 @@
     }
 
     if ($need_scope) {
-      $scope = 'r'.$this->getCallsign();
+      $callsign = $this->getCallsign();
+      if ($callsign) {
+        $scope = "r{$callsign}";
+      } else {
+        $id = $this->getID();
+        $scope = "R{$id}:";
+      }
       $name = $scope.$name;
     }