You would only take CCA on the phone based on its business use. So if you only use the phone 50% for your business, then you would only put $400 as the initial value. The CCA class would be 50.
As to whether you should take CCA on the phone or just expense it, this would be totally up to you. If you use CCA, it will take several years for you to fully expense the phone, and you'd only be getting a relatively small deduction. If your business income is low, that might be fine. But if your business income is high enough, you might what to expense the whole $400 at once rather than dealing with CCA every year.
Related Information:
CCA classes as per the Canada Revenue Agency (CRA)
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