Jia would keep tabs on things both in the muggle world and the wizarding world. Given that she's probably at this point a pretty well known resistance fighter, knowing about her to make contact would be simple. As to how, Jia has a cell phone, one of those late-nineties pieces of crap. She doesn't keep the number much of a secret, half the Muggleborn Witches and Wizards in Britain would know it. 

Although it's dubious if a Pureblood could really figure out how to operate a phone.