diff --git a/app/dao/inbound_numbers_dao.py b/app/dao/inbound_numbers_dao.py
index 4f682710f..e9cdeaed2 100644
--- a/app/dao/inbound_numbers_dao.py
+++ b/app/dao/inbound_numbers_dao.py
@@ -16,13 +16,7 @@ def dao_get_inbound_number_for_service(service_id):
 
 
 @transactional
-def dao_allocate_inbound_number_to_service(service_id):
-    available_numbers = InboundNumber.query.filter(
-        InboundNumber.active, InboundNumber.service_id.is_(None)).all()
+def dao_set_inbound_number_to_service(service_id, inbound_number):
+    inbound_number.service_id = service_id
 
-    if len(available_numbers) > 0:
-        available_numbers[0].service_id = service_id
-
-        db.session.add(available_numbers[0])
-    else:
-        raise IndexError('No inbound numbers available')
+    db.session.add(inbound_number)
diff --git a/app/models.py b/app/models.py
index 80e8ac805..2c4030663 100644
--- a/app/models.py
+++ b/app/models.py
@@ -254,11 +254,11 @@ class InboundNumber(db.Model):
     created_at = db.Column(db.DateTime, default=datetime.datetime.utcnow, nullable=False)
 
     def serialize(self):
-        serialized = {
+        return {
             "id": str(self.id),
             "number": self.number,
             "provider": self.provider,
-            "service_id": self.service_id,
+            "service_id": str(self.service_id),
             "active": self.active,
             "created_at": self.created_at.strftime(DATETIME_FORMAT),
         }
diff --git a/tests/app/dao/test_inbound_numbers_dao.py b/tests/app/dao/test_inbound_numbers_dao.py
index 969dccceb..333ea5d10 100644
--- a/tests/app/dao/test_inbound_numbers_dao.py
+++ b/tests/app/dao/test_inbound_numbers_dao.py
@@ -1,10 +1,11 @@
 import pytest
+from sqlalchemy.exc import IntegrityError
 
 from app.dao.inbound_numbers_dao import (
     dao_get_inbound_numbers,
     dao_get_available_inbound_numbers,
     dao_get_inbound_number_for_service,
-    dao_allocate_inbound_number_to_service
+    dao_set_inbound_number_to_service
 )
 from app.models import InboundNumber
 
@@ -27,8 +28,9 @@ def test_get_available_inbound_numbers(notify_db, notify_db_session, sample_inbo
 
 def test_allocate_inbound_number_to_service(notify_db, notify_db_session, sample_inbound_numbers):
     service = create_service(service_name='test service')
+    numbers = dao_get_available_inbound_numbers()
 
-    dao_allocate_inbound_number_to_service(service.id)
+    dao_set_inbound_number_to_service(service.id, numbers[0])
 
     res = InboundNumber.query.filter(InboundNumber.service_id == service.id).all()
 
@@ -37,18 +39,21 @@ def test_allocate_inbound_number_to_service(notify_db, notify_db_session, sample
 
 
 def test_allocating_a_service_twice_will_raise_an_error(notify_db, notify_db_session, sample_inbound_numbers):
+    from tests.app.db import create_inbound_number
+    create_inbound_number(number='4', provider='mmg')
     service = create_service(service_name='test service')
+    numbers = dao_get_available_inbound_numbers()
 
-    dao_allocate_inbound_number_to_service(service.id)
+    dao_set_inbound_number_to_service(service.id, numbers[0])
 
-    with pytest.raises(IndexError) as e:
-        dao_allocate_inbound_number_to_service(service.id)
+    with pytest.raises(IntegrityError) as e:
+        dao_set_inbound_number_to_service(service.id, numbers[1])
 
     res = InboundNumber.query.filter(InboundNumber.service_id == service.id).all()
 
     assert len(res) == 1
     assert res[0].service_id == service.id
-    assert str(e.value) == 'No inbound numbers available'
+    assert 'duplicate key value violates unique constraint' in str(e.value)
 
 
 def test_get_inbound_number_for_service(notify_db, notify_db_session, sample_inbound_numbers, sample_service):
diff --git a/tests/app/dao/test_services_dao.py b/tests/app/dao/test_services_dao.py
index 2047f0f54..fe03e71bc 100644
--- a/tests/app/dao/test_services_dao.py
+++ b/tests/app/dao/test_services_dao.py
@@ -8,7 +8,7 @@ from sqlalchemy.exc import IntegrityError
 from freezegun import freeze_time
 from app import db
 from app.dao.inbound_numbers_dao import (
-    dao_allocate_inbound_number_to_service, 
+    dao_set_inbound_number_to_service, 
     dao_get_available_inbound_numbers
 )
 from app.dao.services_dao import (
@@ -907,6 +907,6 @@ def test_dao_allocating_inbound_number_shows_on_service(notify_db_session, sampl
 
     service = create_service(service_name='test service')
 
-    dao_allocate_inbound_number_to_service(service.id)
+    dao_set_inbound_number_to_service(service.id, inbound_numbers[0])
 
     assert service.inbound_number.number == inbound_numbers[0].number
diff --git a/tests/app/inbound_number/__init__.py b/tests/app/inbound_number/__init__.py
new file mode 100644
index 000000000..e69de29bb