Hi, Are your ephones configured as dual lines? if not can you delete the ephones involved from CME and recreate it with dual-line mode and see if it makes a difference. instead of:  ephone-dnuse:  "ephone-dn dual-line" when creating the ephone. Thank...

Have a couple more questions, Please confirm below diagram is correct if this is correct your CUCM is sending 488 Media not acceptable, Do you have "MTP Required" option checked in the SIP trunk (If yes remove it), are you using standard SIP profile ...

I would like to see below interaction,