Thanks for posting ArmstrongRJ and welcome to our community.
Please can you expand on what you mean, when you ask
'how do you make overpayments on your mobile phone bill?'
Do you mean you've been charged over and above your tariff price? Do you mean we've overcharged you for your tariff or do you simply mean you're looking to make higher payments to your Virgin Mobile account?
Thanks for confirming, you would need to call our team on 789 from your Virgin Mobile or 0345 6000 789 from any other network and ask to speak to a member of the split contracts team, I hope this helps you.