DISA dial tone problem

I run * 1.4.20 on centos 5, I setup disa access by using Authenticate cmd and then DISA with no password (since DISA with password alone just does not work, gives silence), the problem is that when I get the dialtone and start entering a number the dial tone is not going away, so it sounds weird, a dial tone interrupted by the dialed digits. Everything works though, I can access my pbx remotely via this setup, just this behaviour I don’t get. Is that a bug or a feature? Or I missed something in my setup?