Can I take the customer's number and finish off the platform?

No. Taking work off-platform after a Trady match is a breach of your Worker Agreement and grounds for removal. It also: removes the customer's escrow protection, removes your access to Trady's dispute support if anything goes wrong, removes the audit trail your insurer may need if a claim arises. If the customer wants to re-book you for future work, both of you benefit from staying on Trady — same escrow, same protection, same record.