[asterisk] Fix multiple mailbox support

Mailboxes must be separated with an ampersand; they're currently
separated with commas in the config.

roles/asterisk/templates/extensions.conf.j2

@@ -48,9 +48,11 @@ exten => 700,1,Answer()
     same = n,Hangup()
 
 ; Other extensions
-3-digit extensions dial their associated devices
-exten => _XXX,1,Dial(PJSIP/${EXTEN}, 15)
+; 3-digit extensions dial their associated devices
+exten => _XXX,1,NoOp()
+    same = n,Dial(PJSIP/${EXTEN}, 15)
     same = n,Set(MAILBOXES=${PJSIP_ENDPOINT(${EXTEN},mailboxes)})
+    same = n,Set(MAILBOXES=${REPLACE(MAILBOXES,\,,&)})
     same = n,ExecIf($[${LEN(${MAILBOXES})} = 0]?Congestion(10))
     same = n,Voicemail(${MAILBOXES})
     same = n,Playback(vm-goodbye)