Some steps are necessary in order to do that:

• first provision the users (or use the default sip:alice@open-ims.test or sip:bob@open-ims.test). Make sure than in the HSS provisioning interface in the IM Private Identity configuration, you allow the use of Digest-MD5 for the respective users.
• modify the scscf.cfg and enable the MD5 authorization as the default authentication method instead of AKAv1-MD5.

The Open IMS Core has support for MD5 authentication. Also, you can use SIPp, which has support even for AKA and media

There are several IMS clients out there recently, which can be used with the Open IMS Core.

• FOKUS' OpenIC (Open IMS Client) is available only commercially (Contact: info@open-ims.org). Yet, there is also a free binary OpenIC_Lite version available right here
• The UCT IMS Client which is available under the GPL
• The IMS Communicator, also under the GPL