although IPC is not exactly a 'regular hardware ip phone' it does operate and require configuration the same as one.
that being said, troubleshooting OneWayVoice with IPC is very similar.
please see the following link for one-way-voice troubleshooting:
http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a008009484b.shtml
additional notes:
* be sure ACLs or other network filters are not preventing the voice stream from reaching its destination
* use the IPC built in tools to verify IPC configuration and operation
* if required, plug a hardware phone into the data connection that IPC runs from; configure it with the same DN, partition, CSS, etc; test operation
* verify phone load for IPC; try a different one if needed
* if all else fails, use a sniffer on both ends of the call to help determine what may be happening with the data on the network.
(an excellent freeware sniffer can be obtained from http://ethereal.com)