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If you buy a cell phone, you may not be able to write off the full cost the year of purchase. Instead, you may need to determine its Capital Cost Allowance and claim its cost over a number of years. You can get more information from the website of Tax Tip: Cell phone expenses for self-employed individuals and small business owners
Your phone may be determined as a capital asset and you can claim a Capital cost allowance (CCA) for it. Especially If the phone is very expensive (over $500) you may want to claim CCA on it. Some people use Class 8 (20%), but others use Class 50 (55%), arguing that modern smartphones are basically computers.