Generally, when you have an End User setting up their Okta Profile, if you don't have a source of truth i.e. (AD, LDAP, Workday, etc...). You will not be able to force the end user to complete the enrollment process.
The next time your end users sign in, they are presented with a Please update your profile screen. Users can click Add Phone Number to set up a phone for voice call. or click Remind me later (they are reminded with the same screen after 30 days).