asterisk users - Dec 2009 - Can't place 2nd call to provider

Hello list,

I'm having problems placing the 2nd call via my provider. The first call
goes through and I can talk normally, but when I place the second call,
it doesn't go through and the first call is disconnected. The connection
is 20mbps downstream and 1mbps upstream, so bandwidth is not an issue.

I have another Asterisk running with the same configurations on another
place and with the same provider and I don't have this issue.

What can be the problem?

Best regards,
Paulo Santos