Cfwd no ringing sound

i have 2 phones with the extension 10 + 11 and one Dahdi TE122 ISDN Card

The Phones are Linksys SPA941 (but it is the same with 2 Cisco SPA 303)

In the WebGUI of the SPA with extension 10 i configured a “Call Forward No Answer” to phone 11 after 15 seconds

when I now call from outside: phone 10 is ringing - after 15 seconds it get to phone 11

my problem is that the caller cant hear the ringing sound (after the call was moved from 10 to 11)
(so no tut … tut … tut … on the phone of the customer)

-- Called 10    
-- SIP/10-00000000 is ringing     (everything is fine till here)
-- Got SIP response 302 "Moved Temporarily" back from
-- Now forwarding DAHDI/1-1 to 'Local/11@phone' (thanks to SIP/10-00000000)

any idea would be helpful!