diff --git a/web/src/p2k16/web/core_blueprint.py b/web/src/p2k16/web/core_blueprint.py
index fc245f4..4a13925 100644
--- a/web/src/p2k16/web/core_blueprint.py
+++ b/web/src/p2k16/web/core_blueprint.py
@@ -3,6 +3,7 @@
 import io
 import logging
 import os
+import re
 from typing import List, Optional, Mapping, Iterable, Set, Any, Dict
 
 import flask
@@ -185,6 +186,11 @@ def account_to_json(account: Account):
         "phone": account.phone,
     }}
 
+def is_valid_phone_number(phone_number):
+    if not phone_number:
+        return False
+    number_validator = re.compile(r'\+?\d{8,}')
+    return number_validator.match(phone_number) is not None
 
 def profile_to_json(account: Account, circles: List[Circle], badges: Optional[List[AccountBadge]], full=False, doors=False):
     from .badge_blueprint import badge_to_json
@@ -201,6 +207,7 @@ def profile_to_json(account: Account, circles: List[Circle], badges: Optional[Li
         json["has_door_access"] = authz_management.can_haz_door_access(account)
         json["is_paying_member"] = StripePayment.is_account_paying_member(account.id)
         json["is_employed"] = Company.is_account_employed(account.id)
+        json["has_valid_phone_number"] = is_valid_phone_number(account.phone)
 
     if doors:
         json["available_doors"] = [{"key": door.key, "name": door.name} for door in authz_management.available_doors(account)]
diff --git a/web/src/p2k16/web/static/front-page.html b/web/src/p2k16/web/static/front-page.html
index a4f587f..7e1f8c5 100644
--- a/web/src/p2k16/web/static/front-page.html
+++ b/web/src/p2k16/web/static/front-page.html
@@ -55,6 +55,10 @@ <h4 class="panel-title">Bitraf is open 24/7</h4>
     </div>
   </section>
 
+  <div
+    ng-if="!ctrl.hasValidPhoneNumber" class="alert alert-warning text-center" role="alert">
+    <strong>Please add a valid phone number to your <a href="#!/my-profile">profile</a></strong>
+  </div>
 
   <section ng-if="ctrl.doorsAvailable">
     <h2 class="text-center">Open a door</h2>
diff --git a/web/src/p2k16/web/static/p2k16/p2k16.js b/web/src/p2k16/web/static/p2k16/p2k16.js
index f5deefb..ef28b07 100644
--- a/web/src/p2k16/web/static/p2k16/p2k16.js
+++ b/web/src/p2k16/web/static/p2k16/p2k16.js
@@ -679,6 +679,7 @@
         self.availableDoors = profile.available_doors;
         self.payingMember = profile.is_paying_member;
         self.employed = profile.is_employed;
+        self.hasValidPhoneNumber = profile.has_valid_phone_number;
 
         self.membership_tiers = membership_tiers;
         self.recent_events = recent_events;