Hello there, @SusannaR.

Thank you for joining this thread. I can share some insights about adding customer phone number on the invoices.

Aside from manually editing the invoice template, you can go directly to your customer's profile. From there, you'd add their contact number on the Billing Address field.

I'll show you how:

1. Click Sales.
2. Choose Customers.
3. Click and select the name.
4. Click Edit.
5. Go to the Billing Address field and update the information.
6. Click Save.

All the invoices will follow the new Billing Address information, SusannaR. That way, you no longer have to go through each invoice. 

That's it. Let me know if there's anything else you need about invoices. I'm still here to help you more.