Nimble Group

I have been in arrears with my Zone fitness account..and this has now been handed over to Nimble. I have had numerous calls fom people saying they are from Nimble to my mobile and my employers landline asking me to pay a certain amount to a Standard bank account...but Nimble will not give me an invoice or statement of account...all I get are SMSes with an amount to pay and some banking details. The callers say they cannot send emails. how do I know if this *****ulent or genuine?