diff --git a/app/main/forms.py b/app/main/forms.py
index 9259f5c6f..1229b61e4 100644
--- a/app/main/forms.py
+++ b/app/main/forms.py
@@ -37,10 +37,10 @@ class UKMobileNumber(StringField):
         if self.data.startswith('+'):
             self.data = self.data[1:]
 
-        if not sum([
+        if not sum(
             self.data.startswith(prefix) for prefix in ['07', '447', '4407', '00447']
-        ]):
-            raise ValidationError('Must be a mobile number')
+        ):
+            raise ValidationError('Must be a UK mobile number (eg 07700 900460)')
 
         for digit in self.data:
             try:
@@ -54,7 +54,7 @@ class UKMobileNumber(StringField):
             raise ValidationError('Too many digits')
 
         if len(self.data) < 9:
-            raise ValidationError('Too few digits')
+            raise ValidationError('Not enough digits')
 
     def post_validate(self, form, validation_stopped):
 
diff --git a/tests/app/main/test_phone_number_form_field.py b/tests/app/main/test_phone_number_form_field.py
index a6994fce5..c7a10583a 100644
--- a/tests/app/main/test_phone_number_form_field.py
+++ b/tests/app/main/test_phone_number_form_field.py
@@ -6,62 +6,69 @@ from app.main.forms import UKMobileNumber
 class FormExample(Form):
     phone_number = UKMobileNumber()
 
-phone_numbers = {
-    'invalid': [
-        # Too long
-        '0712345678910',
-        '0044712345678910',
-        '0044712345678910',
-        '+44 (0)7123 456 789 10',
-        # Too short
-        '0712345678',
-        '004471234567',
-        '00447123456',
-        '+44 (0)7123 456 78',
-        # Not mobile (from https://fakenumber.org/generator/freephone)
-        '08081 570364',
-        '+44 8081 570364',
-        '0117 496 0860',
-        '+44 117 496 0860',
-        '020 7946 0991',
-        '+44 20 7946 0991',
-        # Contains non-numbers
-        '07890x32109',
-        '07123 456789...',
-        '07123 ☟☜⬇⬆☞☝',
-        '07123☟☜⬇⬆☞☝',
-        '07";DROP TABLE;"',
-        '+44 07ab cde fgh',
-    ],
-    'valid': [
-        '07123456789',
-        '07123 456789',
-        '07123-456-789',
-        '00447123456789',
-        '00 44 7123456789',
-        '+447123456789',
-        '+44 7123 456 789',
-        '+44 (0)7123 456 789'
+invalid_phone_numbers = sum([
+    [
+        (phone_number, error) for phone_number in group
+    ] for error, group in [
+        ('Too many digits', (
+            '0712345678910',
+            '0044712345678910',
+            '0044712345678910',
+            '+44 (0)7123 456 789 10',
+        )),
+        ('Not enough digits', (
+            '0712345678',
+            '004471234567',
+            '00447123456',
+            '+44 (0)7123 456 78',
+        )),
+        ('Must be a UK mobile number (eg 07700 900460)', (
+            '08081 570364',
+            '+44 8081 570364',
+            '0117 496 0860',
+            '+44 117 496 0860',
+            '020 7946 0991',
+            '+44 20 7946 0991',
+            '71234567890',
+        )),
+        ('Must not contain letters or symbols', (
+            '07890x32109',
+            '07123 456789...',
+            '07123 ☟☜⬇⬆☞☝',
+            '07123☟☜⬇⬆☞☝',
+            '07";DROP TABLE;"',
+            '+44 07ab cde fgh',
+        ))
     ]
-}
+], [])
+
+valid_phone_numbers = [
+    '07123456789',
+    '07123 456789',
+    '07123-456-789',
+    '00447123456789',
+    '00 44 7123456789',
+    '+447123456789',
+    '+44 7123 456 789',
+    '+44 (0)7123 456 789'
+]
 
 
-@pytest.mark.parametrize("phone_number", phone_numbers['valid'])
+@pytest.mark.parametrize("phone_number", valid_phone_numbers)
 def test_phone_number_accepts_valid_values(phone_number):
     form = FormExample(phone_number=phone_number)
     form.validate()
     assert form.errors == {}
 
 
-@pytest.mark.parametrize("phone_number", phone_numbers['invalid'])
-def test_phone_number_rejects_invalid_values(phone_number):
+@pytest.mark.parametrize("phone_number, error_message", invalid_phone_numbers)
+def test_phone_number_rejects_invalid_values(phone_number, error_message):
     form = FormExample(phone_number=phone_number)
     form.validate()
-    print(phone_number)
-    assert form.errors != {}
+    assert form.phone_number.errors[0] == error_message
 
 
-@pytest.mark.parametrize("phone_number", phone_numbers['valid'])
+@pytest.mark.parametrize("phone_number", valid_phone_numbers)
 def test_phone_number_outputs_in_correct_format(phone_number):
     form = FormExample(phone_number=phone_number)
     form.validate()