i have engaged some of these so called debt collectors what i found was once they collected there commision they didn care about the debt anymore
in fact one company promised this that and other ,i paid there fee monthly by direct debit they were to collect 30k for me i paid them about 2k they collected in €1300 and tried to sue us then for the commision on what they didn collect! Total waste of time ,call personally to your customers you may find they are in difficulty ,work out a scheme of payment it often works out
and they will in turn be a good loyal customer in the future