Contract signed in 2016 still not paid after full service completed

I have communicated in person to CEO Mr Antonio Da Silva who signed a contract for our company to provide internet services/ online marketing a online design and placement for their company. Since then the company has some new people in place. They are refusing to pay for these services already rendered by us. At first, it was that we dont know if you actually did these services on our behalf. We then provided proof of a signed contract and a unpaid invoice sent. I am now in contact with both an administrator Mr Mario Lironel and Mr Belamino Vandanum who have both been stringing us along again. Mr Mario said to me three weeks ago that he was awaiting signatures from Mr Belamino and we still waiting as they continue to keep us waiting. Mr Belamino said to me on Thursday that he does not think that this is something they needed. For a director of a company to say something like that is extremely not sensible. I did tell him that this service has been done and utilised by his company almost a year and a half ago and its good business etiquette for him to pay his bills but he just acted unconcerned and not interested in sorting this out. He kept asking me to call again. Miss Ana Da Silva had confirmed via email to them that we did in fact finish our part and that they needed to pay us but he still was oblivious to the situation. And so I warn fellow business owners to rather get customers to make payment beforehand or take deposits before we do all the work for nothing because I have now found evidence that even some large businesses are in fact operating very dishonestly and not taking responsibility for services they use. Do contact me if you need me to send these emails to you to validate my review about Apiex Angola and the lack of trustability