A Thai court is expected to rule on Friday whether the recently suspended prime minister, Paetongtarn Shinawatra, can return to office, a decision that could usher in a new period of political instability in Thailand.

The verdict from the Constitutional Court in Bangkok comes nearly two months after it suspended Ms. Paetongtarn over allegations of ethics violations. The charges stem from a private phone conversation she had this year with the Cambodian leader Hun Sen, which fueled anger when it became public because it raised questions about her competence and loyalty.

Ms. Paetongtarn’s case is seen as an important test of her powerful family’s standing with the royalist-military establishment in Thailand, which is a constitutional monarchy. Her father, Thaksin Shinawatra, a former prime minister who has long been a foil for members of the country’s old guard, is widely thought to have made a grand bargain with them that allowed him to return to Thailand in 2023, after years in exile, and essentially avoid jail time.

Now Ms. Paetongtarn, 39, who became premier last year, faces the prospect of being removed from office and barred from politics.

On June 15, she and Mr. Hun Sen had a phone call about a tense border dispute, which later in the summer exploded into deadly clashes.

Soon after the call, Mr. Hun Sen posted a recording of the discussion on social media. In it, Ms. Paetongtarn addressed Mr. Hun Sen as “uncle,” seeming to be deferential to him and critical of Thailand’s military. Demonstrations broke out in Thailand, with protesters calling for her resignation.