Until 3CX V15 SP5 we used a backup SIP Trunk from our VoIP provider. The idea is that we add 2 SIP Trunks both with the same DID’s. From 3CX V6 till 3CX V15 SP5 was this working fine. When there was a very brief disruption, the end user didn’t notice this because the backup account was taking over the same DID. Suddenly, at the release of SP5 of version 15 this way of backup isn’t working anymore. Hope there are a lot of people to find this a very smart way to back up the VoIP provider so this will work again at SP6 of version 15.