Timeout ringing on the sip phone

In the dial plan, when we call a sip phone the timeout is 0, and we expect that the phone ring indefinitely if nobody pick up it.
The DialTimeout is put to 0 in the extensions.conf file : n(DoDial),Dial(${DialNumber},${DialTimeout},g)

The trace in asterisk

– SIP/103-000000a4 is busy
== Everyone is busy/congested at this time (1:1/0/0)
– Executing [s@action:10] GotoIf(“SIP/184-000000a2”, “0?AttenteDial”) in new stack
– Executing [s@action:11] GotoIf(“SIP/184-000000a2”, “0?FinDial”) in new stack
– Executing [s@action:12] GotoIf(“SIP/184-000000a2”, “1?FinDial”) in new stack
– Goto (action,s,17)
– Executing [s@action:17] Hangup(“SIP/184-000000a2”, “”) in new stack

However, after few minutes, is hung up. I don’t understand why the SIP phone 103 is busy ?
Have you an idea to resolve the problem ?

Any help or guidance is much appreciated.
Thanks.