SIP trunks do not fall through to next trunk

I have registered with two voip providers.
We have built an Ourbound Route with the Trunk Sequence containing these two SIP trunks.
When we place a call while the first voip (SIP) provider is busy, we get a message from the operator of the first provider, that we cannot use the account, which is correct.
How can we overcome this problem.
The problem does not appear when we use our ISDN trunks.

If the provider sends back a busy, then it is not going to fail over. They need to send the equivalent of a congestion in the signaling. Some providers don’t play right here. If it is just a matter of hitting the maximum channels, then use max channels. If they count inbound also, then use the context that is automatically generated with each trunk in it’s inbound configuration to automatically count the inbound channels being used as well.