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.
• Next just make sure that the client is using the P-CSCF address (sip:pcscf.open-ims.test:4060 by default) as strict outbound proxy and the REGISTER Request-URI is "sip:open-ims.test" (or your own domain name) and not "sip:pcscf.open-ims.test:4060" as many SIP clients fail here.