Cisco Call Manager 9 extension to Asterisk trunk integration

I need have this integration
Could someone help me with it?

an incoming call (in the Cisco trk) need to end in one extension that have to enter in an Asterisk switch as a trunk.

I can not do a trunk to trunk connection between Cisco and Asterisk
I need an extension to trunk

PSTN —>Cisco—extension—>Asterisk-trunk

I appreciate the help you can provide