Help with Leaving voicemail without having phone ring

Hello,
Ive seen people call them backdoor numbers however Im not sure if that is exactly what I am looking for. Basically as briefly as possible I will explain what I am trying to accomplish and hopefully someone out there will know how it is done....

To be blunt, I am trying to leave a message for someone who I believe has AT&T wireless. Anytime I speak to him I get stuck on the phone for ridiculous lengths of time (you cant hangup with this guy). I do need to keep in contact though. So I am trying to find a direct number to dial that will let me enter his cell phone number, which will then give me the option to leave him a message.... Unless there is another way to leave a message on his voicemail.
My biggest obstacle is I do not have AT&T. I have Verizon wireless. I would settle for the same chance to leave a vm on his AT&T landline as well.
Does anyone know how to do this?
Thank you

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› See More: Help with Leaving voicemail without having phone ring

You are looking for the guy's voicemail deposit number. You call that number and are asked to input the customers 10 digit wireless number, from there you can leave a message. Easiest way to go about getting it may be to make up some lame excuse to the guy (uh, for some reason you can't leave him a message the regular way) and have him retreive it for you, unless you know the last four digits of his SSN.