Question: If a person is currently in the middle of shemone esreh, and the tzibbur are up to kedusha is it acceptable just to listen to kedusha, or should I try to finish the shemone esreh quickly in order to answer kedusha with the tzibbur?
Answer: We see in Shulchan Aruch Siman 109 (Se’if 1) that a person who arrives late to a minyan (i.e., after the tzibbur have started to daven the shemone esreh) should not begin to daven the shemone esreh unless he will be able to finish davening before the tzibbur reach kedusha. We see from this that answering kedusha verbally is ideal. Therefore, if you are able to daven slightly faster in order to answer kedusha verbally – this is the preferred option, and you should do so.
We also see in Shulchan Aruch Siman 122 (Se’if 1) that if a person is currently in the middle of e-lokay netzor and the shli’ach tzibbur reaches (or is about to reach) kedusha one should try to quickly finish e-lokay netzor in order to answer kedusha with the tzibbur.