Are you using H.323 or MGCP on your 3745? If H.323, make sure you have a dial-peer that routes calls to CallManager. Something like the following, where PBX phones would be calling 5xxx extensions on the CallManager at 1.1.1.1
!
dial-peer voice 3100 voip
destination-pattern 5...
session target ipv4:1.1.1.1
dtmf-relay h245-alphanumeric
fax rate disable
no vad
!
If it's MGCP, is the gateway configured with an appropriate CSS?