Fix voice mailbox number reading from SIM

The MBDN elementary file needs to be used instead of VMBX to fetch the
voice mailbox.

TS 151.011 contains following information:
"10.3.43 EFMBDN (Mailbox Dialling Numbers) This EF contains dialling
numbers to access mailboxes associated with Voicemail, Fax, Electronic
Mail and other messages. It may also contain associated alpha-tags for
each supported mailbox. Each dialing number shall be associated with a
message waiting indication group type using EFMBI (see 3GPP TS 23.038
[12] for message waiting indication group types)."
diff --git a/audio/telephony-maemo.c b/audio/telephony-maemo.c
index a2bbd0f..f8da349 100644
--- a/audio/telephony-maemo.c
+++ b/audio/telephony-maemo.c
@@ -1824,7 +1824,7 @@
 		return;
 	}
 
-	pb_type = SIM_PHONEBOOK_TYPE_VMBX;
+	pb_type = SIM_PHONEBOOK_TYPE_MBDN;
 	location = PHONEBOOK_INDEX_FIRST_ENTRY;
 	location_type = SIM_PHONEBOOK_LOCATION_NEXT;