I believe the answer to your question is laid out in the following configuration guide:

http://www.cisco.com/en/US/docs/voice_ip_comm/unity_exp/rel3_2/administrator/voicemail/12advVM.html#wpmkr1025284

Configuring Integrated Messaging

Follow this procedure to configure the Integrated Messaging capability.

Prerequisites

The system must have a default security certificate and private key  installed before SSL connections are permitted on Cisco Unity Express.  Use the show crypto key command to display  the system default certificate-key pair. If no default pair exists,  follow the procedure in "Configuring Security"  on page 295.

Required Data for This Procedure

Name of a subscriber group that has the vm-imap privilege.

SUMMARY STEPS

1. config t

2. service imap

3. enable

4. maxsessions num-sessions

5. session idletimeout minutes

6. session security {ssl | none | mixed | keyLabel labelname}

7. enable

8. no enable

HTH.

Regards,

Bill

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