An SR22 pilot took off without clearance because he misunderstood the Controller's command and was notified about his error by ATC once airborne.

Narrative

I was cleared to taxi to the runway. When approaching the hold short line for the runway I switched to the Tower frequency. The Tower Controller called me to ask if I would be 'ready to go'. I answered I was ready and then the Controller issued an instruction; which I misunderstood to mean I was cleared for take-off. He also added traffic would be landing on the other runway. I read back 'cleared for take-off; runway XX; will be looking for the traffic' and then proceeded to take-off from the runway. When I was climbing the Controller told me I was expected to line up and wait because of a landing traffic on the other runway (an intersecting runway) and gave me a phone number to call when back on the ground. As far as I remember I did not hear him state 'line up and wait' - maybe he was cut-off or he pressed his mike after he had started to speak or I just plainly misunderstood him. I was (wrongly) convinced I was cleared for take-off and this is actually the instruction I repeated; but the Controller did not catch my incorrect read-back. I believe the reason for the miscommunication is that I was really expecting to be cleared for take-off when the Controller asked me if I was ready to go so close to the runway hold short line and my mind was already set. Also when he said there would be a landing traffic on the other runway I (wrongly) assumed the traffic would be landing well after I was airborne. In addition; the Controller did not catch my incorrect read-back.