The bigger problem is, of course, that you need speech data with (fairly) accurate transcripts for training ASR systems. These typically don't exist for call center calls.
Transcriptions are not really the issue here, the cost of freelance transcribers is relatively low. It is privacy that makes it so hard, most of the call center calls need to have some kind of user authentication, which means they would need to be anonymized prior to being transcribed and used as a training material.