Add information about pricing to inbound SMS page

One of the things that came out of the support analysis was that people
were asking how much inbound SMS costs. There wasn’t a significant
volume of these requests, but the fix seems low-effort and
non-disruptive enough that we should do it.

Content by Thom.

app/templates/views/service-settings/set-inbound-sms.html

@@ -27,17 +27,16 @@
         {% endif %}
       {% else %}
         <p>
-          Receiving text messages from your users is an
-          invitation&#8209;only feature.
+          If you want to be able to receive text messages from your users, please
+          <a href="{{ url_for('.support') }}">get in touch</a>.
         </p>
         <p>
-          If you want to try it out,
-          <a href="{{ url_for('.support') }}">get in touch with the GOV.UK Notify team</a>.
-        </p>
-        <p>
-          We’ll set you up with a special phone number, and you’ll be able to see
+          We’ll create a special phone number for your users to contact. You'll be able to see
           the messages on your dashboard, or get them using the API.
         </p>
+          It doesn’t cost you anything to receive messages. Users will pay their normal rate for
+          messaging a mobile number.
+        </p>
       {% endif %}
       {{ page_footer(
         back_link=url_for('.service_settings', service_id=current_service.id),

tests/app/main/views/test_service_settings.py

@@ -1821,7 +1821,7 @@ def test_cant_resume_active_service(
         'main.service_set_inbound_sms',
         ['sms'],
         (
-            'Receiving text messages from your users is an invitation‑only feature.'
+            'If you want to be able to receive text messages from your users, please get in touch.'
         )
     ),
     (