"Back home" usually refers to a country. Therefore, if Jonny is abroad (say in China) but is from England then the phrase "expecting a call from his business partner back home" would suggest a phone call from England. "Back home" can also be used to refer to cities/towns: it just depends on context. If the phrase "business partner back home" is used and the subject is in his home country then it would usually refer to his "home town" so to speak. "Back home" could actually be used to refer to a phone call from someone's family at home. Because your sentence was in a "business context" that is unlikely to be the case here. -- Good luck and take care :)!