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

No. Taking work off-platform after a Get Trady match is a breach of your Worker Agreement and grounds for removal. It also: removes the customer's payment protection, because their money is no longer held in escrow by Stripe, removes your access to Get 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 Get Trady — same escrow, same protection, same record.