I don't use one. The groundline at home works just fine for my internet use & very rare voicemail messages.
I don't see the point to the douchebags that need to call people while taking a shit, driving, in church, at the movies, on a date, et al.
There's the one young (Senior High School-age) douchebag (Mark,) I deal with a few times a week, at work, that needs to be fucking working at closing-time rather than stopping (every time it rings, as though he's a paramedic, lawyer, or something,) to yak to whomever it is that needs to get a fucking clue about the tit-head's evening priorities.
I've taken the damn thing away from him once already. I've bitched to my supervisor about him ignoring me when I tell him to shut it the fuck off as we need him clearing cardboard extensions, as I'm Senior Staff.
In addition to the fact Mark The Tithead needs to let the call go through to voicemail until he's done helping me or the other kid, Isagami, bring everything inside and call whoever it is back, but only once he's officially off-the-clock.
After all, Isagami & I both catch shit from one of our assistant managers/supervisors, if we're wearing I-pod/MP3-equipped cell headphones, with the music on Or off.