diff --git a/lib/models/voip-carrier.js b/lib/models/voip-carrier.js
index 98a3b5a..7a31d96 100644
--- a/lib/models/voip-carrier.js
+++ b/lib/models/voip-carrier.js
@@ -12,6 +12,9 @@ class VoipCarrier extends Model {
   static _criteriaBuilder(obj, args) {
     let sql = '';
     if (obj.account_sid) {
+      // carrier belong to an account when
+      // 1. account_sid is set
+      // 2. account_sid is null and service_provider_sid matches the account's service_provider_sid
       sql += ` AND (vc.account_sid = ? OR
         (vc.account_sid IS NULL AND vc.service_provider_sid IN 
           (SELECT service_provider_sid FROM accounts WHERE account_sid = ?))