How can I capture mobile numbers of customers who pay in cash and get reports on it?

First, You need to mention the customers mobile numbers in Billing/shipping window ,So that we can capture it and get report by adding custom column of that field and to mention it follow below steps:
Step 1: Go in administration and select configuration.
Step 2: Then select voucher series configuration.
Step 3: Then under it select desired voucher type and series.
Step 4: Then click on voucher configuration.
Step 5: Then enable the option of seperate Billing/Shipping.
Step 6: Save the configuration and go back.
Step 7: Go in transaction and create desired voucher.
Step 8: Then after select party , billing/shipping window will appear mention the mobile number and other required details and save the voucher.
To add custom column of that field follow below steps:
Step 1: Take backup of your data.
Step 2: Then go in display section and select account books.
Step 3: Then select account register (Standard).
Step 4: Then select sale regitser and select other given options as per your requirement.
Step 5: Select desired date and other configuration and open the report.
Step 5: Then after open the report, on right hand top there is an option of custom column.
Step 6: Then click on it and click on add format mention name of format.
Step 7: Then Click on add new column and mention heading name position, alignment type and select data field category as inventory voucher (header).
Step 8: Then click on fields and select filedof billed_party_mobile_no and save field and format.
Step 9: Then go back and re open the report.