The transfer that fails what type of circuit is it, PRI, T-1, analog trunk? Are you doing a blind transfer? If you stay on the line and wait for the called party to answer does it work? You may need to adjust ASYD 156 and 157 usually 31 is set but I have changed it to 1F in the past to clear up similar issue, this should not matter if you are using PRI. Another test would be to make ARTD CDN 28 a 0 and try the transfer but I would not leave this setting as it can lead to trunks not releasing. One more thing the route that fails check ARSC make it all 1 and test, is the RSC of the phone not allowed to use that route?