The phrase "if so" is commonly used in conversations and written communications to seek clarification or confirmation of a previous statement. However, there are several synonymous phrases that can be used interchangeably, including "in that case", "if that's the case", "if that's true", "so", "therefore", "thus", and "consequently". These phrases help to convey the same meaning as "if so" and can be used in different contexts to add variety and clarity to the communication. Choosing the right synonym for "if so" depends on the tone, context, and intended audience of the communication.