This is a regular verb. It follows the conjugation pattern of the verb falar.
In Brazilian Portuguese, ligar is more commonly used in everyday speech. Telefonar is correct but sounds more formal.
Present:
A Ana telefona para a avó todos os domingos.
A Ana telefona à avó todos os domingos.
Ana calls her grandmother every Sunday.
Preterite:
O Miguel telefonou para o cliente ontem.
O Miguel telefonou ao cliente ontem.
Miguel called the client yesterday.
Conditional:
A Clara telefonaria para você mais tarde.
A Clara telefonaria mais tarde.
Clara would call you later.