Is it true "static" where you always hear them speak but there is a crackling noise along with it, or does their voice sound like it's run through a synthesizer and/or blank spots throughout the conversation?
If it's the first, then it's usually caused by an analog device -- either bad wiring on their voice service or an analog cordless phone or loose RJ11 jack it's plugged into. Ask them to try a different, corded phone.
If it's the second, then it's probably a digital problem. If it's happening at the same time, there may be a bandwidth issue if they're using VoIP service instead of analog. The problem is it could be a number of things depending their equipment.
1. Ask if they have regular (analog/POTS) or digital VoIP (Vonage, Skype) home voice service
2. Ask if their cordless phone is digital (most made in the last 10+ years are). They could also have a frequency clash with another device -- primary suspect being WiFi (800MHz?).