asterisk users - Jun 2007 - On multiple dial phones continue ringing after picked up

Hello,

I'm using a dial command to make several phones ring. I use this format :
Dial(SIP/4029&SIP/4030,15,tTr)

As soon as one of the phone is picked up, all the others should just stop
rining.

But the fact is that they continue to ring for several seconds (4-5s), and
this is quite annoying as all phones are in the same room. Phones are Siemens
Gigaset C450IP and Linksys SPA921.

Do you have any idea what I could do to fix this ?

Regards,

Yves.