The simple answer is, yes. A couple of variables need to be addressed when using VoIP behind NAT. First and foremost what protocol of VoIP are you using H.323 or SIP? If you use SIP then NAT is not a problem. SIP was designed with NAT in mind and supports NAT natively.
The older versions of H.323 have problems with NAT because the originating devices IP address (say 192.168.1.100) is actually used in the higher layers and doesnt get translated, therefore the terminating devices wont be able route back to it.
This problem can be addressed with the use of a PIX or other H.323 aware firewall. You can also address the problem with the use of a H.323 proxy that mediates H.323 call setups between your private and public IP addresses.
There was a link to a Cisco PDF file floating around this forum that specifically addressed this scenario.
Hope that helps