Three-way call a voicemail box

My medical industry call center is calling out and when we get a verbal authorization for a prescription, we are required by law to have it in a voicemail box if the pharmacist isn’t available. The problem is at times, the person transferred to the voicemail doesn’t give us all of the required information. I need to set up a system where we can transfer someone to a voicemail and still be on the line with them while they record their message. That way we can be sure all of the information we require is supplied. I’ve already asked “we have all the calls recorded already, why do we need this?” and the answer is “It has to go into the pharmacist’s voicemail and that voicemail’s greeting is supposed to say certain things. The law requires it.” Can I add a voicemail to a call already in progress?
CentOS kernel 2.6.18