Unitel sued dos Santos' Dutch company Unitel International Holdings (UIH) in 2020 over loans that were provided to fund UIH's acquisition of shares in telecoms companies.
The loans were not repaid and around $395 million plus interest remains outstanding, Unitel's lawyers told London's High Court on Tuesday.
Unitel is applying to personally add dos Santos – Africa's first female billionaire, whose father Jose Eduardo dos Santos ruled Angola for 38 years until 2017 – to its case against UIH.
The company alleges dos Santos, who was a director of Unitel when the loans to UIH were made, breached her duties to Unitel by failing to take steps to ensure UIH repaid the loans.
However, dos Santos' lawyers argued in court filings that Unitel is responsible for UIH's inability to pay, because of its alleged role in the "unlawful seizure by the Angolan state of UIH's assets." Unitel denies the allegation.
Dos Santos has faced corruption accusations for years, including allegations by Angola that she and her husband steered $1 billion in state funds to companies in which they held stakes during her father's presidency, including oil giant Sonangol.
Around $1 billion of her assets in Angola were seized last year, while other assets linked to her have been seized in Portugal.
Global police agency Interpol also issued an international arrest warrant against dos Santos in November.
Dos Santos, who says she lives in Dubai, has repeatedly denied any wrongdoing and her lawyers said on Tuesday that the allegations against her are part of a "political conspiracy".
She "vehemently denies that she has breached any of her director's duties," her lawyer Richard Hill said in court filings.
Judge Mark Pelling is expected to rule on Unitel's application to add dos Santos to the case at a later date.