How to let cell phone join a queue

Hi, we got a requirement for call center. For a queue, If nobody picks up the call, transfer to a cell phone. Is there a way to achieve this on asterisk? or how to let the cell phone join the queue?

I have a gateway with 4 FXO and 4 FXS? the 4 FXS are used as extensions to join the queue.
Thanks for help.

Dave

Use a timeout on the queue, and fall through to a Dial to the mobile, or use queue penalties so that the mobile phone only becomes eligible to receive calls after some time.

I tried the penalty, but channels with higher penalty never get ringing.

queues.conf
[queue1]
member => SIP/8000,10
member => SIP/8001
member => SIP/8002

SIP/8001 and SIP/8002 took turn ringing. but SIP/8000 never rang.
I will try the other method you mentioned.

exten => 1,1,Queue(Sales,60)
exten => 1,n,Dial(SIP/gateway/xxxx)
exten => 1,n,Hangup

Thanks.
Dave