Hello,
Just my personal opinion...
Next time it happens, use standard procedure...
Give a readback ... Roger climb to FL90... (or even CONFIRM climb to FL90).
This will give the ATCO the possibility to REPLY (yes/No/ Affirm due to mil tfc or xxxxx) If you get a a heterodyne ... you know 2 ATCO's or 1ATCO and someone else are TX on the same frequency...
If you get a response NEGATIVE inform immediately the ATCO concerned that unauthorised TX is in progress. This will give the ATCO the possibility to closely monitor the TFC/FREQ (if necessary he'll ask the SUPV to keep a listening watch on the freq, without squelch)...
Such cases require immediate action from both sides...
Tell your captain that he was wrong, immediate action to advise the ATC is necessary.
And ... file an ASR...
Greetings...
BM
PS: Don't use your TCAS as a separation tool .... I can give you hundreds of reasons why it isn't made for that !
Just my 2cents ...