Have SV8500 connecting to SV8300 via IP CCIS. SV8500 ITL-12D set can call the SV8300 ITL-6D set. The SV8300 set will ring and when answered there is speech both ways between the SV8500 and SV8300. If the SV8300 set attempts to call the SV8500 set there is silence, no connection ever made and then drops. Have reviewed all programming multiple times and seem to be missing something. Simple connection SV8500 is PC1 and SV8300 is PC2.