Miss Weinstock is a regular customer to Express Car Service, Roman's taxi firm, and is mentioned in the beginning cutscene of the mission Jamaican Heat. She is never seen nor heard while on the phone to Roman.
While Roman is enraged at Mohammad, Miss Weinstock calls the cab service looking to be picked up. Roman, thinking that it is Mohammad calling again, flips out over the phone, whilst finding out that it was Miss Weinstock. He calms down nervously, obviously meaning that Miss Weinstock is an important, or demanding person, probably of old age.