i want to be able to call voicemail on a 5s thru my cars (mercedes) bluetooth system

One way that might work would be the similar to the way you can check your voicemail from a different phone (i.e., a phone not your own). If you dial your own cell phone number from your own phone (at least with AT&T & Verizon) it will go directly to voicemail and prompt for your voicemail password -- which you may have to first research since that password is never prompted for when you check voicemail from your own phone the regular way. I haven't tried this with my Mercedes to see if I can then enter the password from the dashboard Command system keys.