This is probably because of the nature of the codec used. G.729 is an ACELP codec, which is a high compression codec. In addition, the codec is designed mainly to handle human speech. This could have an impact on Abacus when testing Voice Quality Measurement using PSQM or PESQ. Abacus is sending a special tone called "pilot tone" or "syn tone" every time a PSQM or PESQ measurement is made. This tone is sensitive to high compression codec like G.729. If the call is routing though an IP network, it is likely that other network impairments could further affect the successful recognition of the tone. When Abacus fails to detect the tone, it will report an Path Confirmation error.
This same error could happen on other high compression codec like G.722.
For trouble shooting, users may compare the sending pilot tone and wave file by using the QOM capture or media monitor.
The solution to this problem: users may fine tune their network to reduce the sources of network impairments. if that is not possible, the only option is to use a codec with lower compression such as G.711.