Can I show the customers telephone on invoice?
There are variables such as:
{{ customer.telephone }}
{{ customer.mobile }}
{{ customer.fax }}
but you would need to use custom invoice template which requires some HTML knowledge.
I’m planning some improvements to custom fields which would allow you to inject extra information without HTML knowledge.
Where to edit invoice template?
Go to Settings and then View Templates.
Create new template or use existing one if present.
Bear in mind that currently some options, available in build-in template are not available in custom template without knowledge of HTML.