No invoice despite requests

I have received sms's from Lancet telling me that my medical aid rejected a claim and it is due by me. They sent me an sms after which I called them and asked for an invoice so I can pay. No email. Then they sent more sms's and I called again - at least 3/4 more times asking for an invoice - they said they would send it. I will not pay based on an sms as I need invoice for proof and tax purposes. To date i have not received an invoice. Now I got an sms threatening to hand me over. I have provided my email address on at least 4/5occasions, and they wont post it to me - stating they only sms or email. Well I haven't got an email from them and now they want to hand me over for collection. I am very tired of calling them asking for the same thing all the time! I WILL not pay unless I have an invoice!