If they have an iPhone, you can go into contacts and turn on emergency bypass just for you and that way their phone will ring when you call but not anyone else.
Or, if you have an aPhone, you can ssh directly into it (ssh server apps don't require root), or make it do an additional action (such as turning off silent mode, or playing something) when you call (I'm sure automation apps can do this without root), or a million other creative ideas as root user.
