Hi Ray,
There may be a better way to achieve this but here's my idea. even without integration to AD, you can just duplicate the enduser profile that has his DN, add Mobile description on it and replace the DN by his mobile number (valid to route pattern)
both of these profile, DN and mobile will appear in corporate directory;
ex
John doe - DN
John doe - Mobile
I tried to check your requirement on the net but I only see 1 number appear in the ip phone display (corp directory). But I guess i wasn't looking that much.
goodluck on this and please post your best solution.
Regards,