Most Cisco documentation specifies that RTP & RTCP traffic will use a dynamically chosen port number in the range 16384 to 32767, with RTP using an even port number & RTCP using the subsequent odd numbered port. However as of IOS XE 3.10.2 the 4000 series routers actually use the range 8000 to 48200 by default, fortunately this information is in the release notes. This change means that any ACLs that restrict traffic based on the 16384 to 32767, or firewalls that aren't H323, MGCP, SCCP or SIP aware may block the RTP audio packets.
If you're unable to get the ACL or firewall configuration updated, then as a workaround you can force the 4000 series router to use the same port range as older Cisco routers:
voice service voip
rtp-port range 16384 32766
Note 32766 as the maximum as 32767 would be used for RTCP.