[mobile-broadband-provider-info] Include information for voicemail on AT&T USA

[Thomas Haller <thaller src gnome org>]

commit 265915f3584a50dd53c6615f799b3357a0b91ef1
Author: Chris T <chris talbothome com>
Date:   Mon Jun 28 20:58:26 2021 -0400

    Include information for voicemail on AT&T USA
    
    https://gitlab.gnome.org/GNOME/mobile-broadband-provider-info/-/merge_requests/32

 serviceproviders.2.dtd | 8 +++++---
 serviceproviders.xml   | 5 +++++
 2 files changed, 10 insertions(+), 3 deletions(-)
---
diff --git a/serviceproviders.2.dtd b/serviceproviders.2.dtd
index 347378a..cc03238 100644
--- a/serviceproviders.2.dtd
+++ b/serviceproviders.2.dtd
@@ -34,11 +34,13 @@
 
 <!ELEMENT visual-voicemail (standard*,
                             destination-number*,
-                            carrier-prefix)>
+                            carrier-prefix?)>
 
 <!ELEMENT standard EMPTY>
-<!ATTLIST standard type (cvvm|otmp) #REQUIRED>
-<!-- Whether the carrier supports OTMP or CVVM for Visual Voicemail -->
+<!ATTLIST standard type (cvvm|otmp|proprietary) #REQUIRED>
+<!-- Whether the carrier supports OTMP or CVVM for Visual Voicemail
+     Some carriers impliment their own proprietary standard.
+-->
 
 <!ELEMENT destination-number (#PCDATA)>
 <!-- This is the destination number to activate/deactivate Visual
diff --git a/serviceproviders.xml b/serviceproviders.xml
index 566014d..b2ef93a 100644
--- a/serviceproviders.xml
+++ b/serviceproviders.xml
@@ -13209,6 +13209,11 @@ conceived.
                        <network-id mcc="310" mnc="410"/>
                        <network-id mcc="310" mnc="560"/>
                        <network-id mcc="310" mnc="680"/>
+                       <voicemail>+17249209990</voicemail>
+                       <visual-voicemail>
+                               <standard type="proprietary"/>
+                               <destination-number>94183567</destination-number>
+                       </visual-voicemail>
                        <!-- 
http://www.wireless.att.com/answer-center/main.jsp?solutionId=35078&t=solutionTab -->
                        <apn value="wap.cingular">
                                <plan type="postpaid"/>