I want the sales invoice to print the customer's mobile number. How can I configure the invoice to print custom fields like mobile number?

To print the customer's mobile number in the sales invoice, there is no need to configure custom fields for the mobile number. You need to follow the below steps to print the customer's mobile number in the sales invoice:
First, confirm whether the invoice format is standard or designed.
If using standard format, follow the below steps:
Step 1: Go to Administration, then select Configuration.
Step 2: After Click on the Configuration option within the Administration section.
Step 3: You will find the invoice or document printing.
Step 4: In the Invoice Document Printing window, choose the Standard tab.
Step 5: Select Sales Invoice. In the Sales Invoice window, find the field Print Party Details and set it to Yes.
Step 6: Once you have enabled the Print Party Details option, a configuration window will appear.
Step 7: Within the configuration window for party details, you will see a field for Print Mobile No. Tick or check this field to instruct the software to print the mobile number.
Step 8: By following these steps, you will configure the printing of party details on a standard sales invoice. Make sure to enable the Print Party Details option and check the Print Mobile No.
or
If the designated format is used, then you want to include a customer's mobile number on a sales invoice, and you have a specific format to follow. Here's how to do it:
Step 1: Go to the Administration section.
Step 2: Then, click on Configuration.
Step 3: Within the Configuration menu, select Invoice or Document Printing.
Step 4: After that, go to the Advanced settings.
Step 5: Next, choose Sale Invoice.
Step 6: Now, select the particular sales invoice format that you need.
Step 7: To add the customer's mobile number to the sales invoice, press the Alt+F4 key. This will take you to the specific place on the invoice where the party's mobile number should be.
Step 8: When you do this, the customer's mobile number will be printed on the sales invoice in the designated location, as per the format you've chosen.