You must follow the TCAS, the way I understand how the system works, is that if two acft are on a collision course, the TCAS systems working with the acft transponders, work out the resolution and talk to each other. They will tell one acft to climb and the other to descend. You have 5 seconds to respond and you must attain the commanded ROC/ROD with 5 secs. If the TCAS reverses the command then you have 2 or 3 secs to follow the reversal this could put the CA on the ceiling or the coffee on the floor. If you don't follow the TCAS then you end up with the Bodensse accident. The TCAS system doesn't consult with ATC, thats what the pilot is there for and there are standard phrases to use when the situation occurs.
Someone may have a better understanding of it than I have, but I'll be following the TCAS even if it is contradicts an ATC instruction.