I am sure that if you word it along the lines of;
On purchasing this product/service warranties are guaranteed, callouts however are only covered if your monthly payments are current, should you fall into arears an up front payment will be required before a 'call out' will be effected. Then sting them if they are in arrears.
So really you are saying 'I fully guarantee this product/service but if you don't pay, I aint coming to fix it'
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