Is there a way to feed an audio input into a phone?

[Cowgoesmoo2]

Hi. My job's been having me call many different places and usually leave these long messages and it's killing my voice.
Is there some way for me to feed audio input into the phone in order to leave a voicemail?
I'm using a flip phone they provided me, but in order to save my voice I'd be fine with using my iPhone if it is so required. So to restate my needs, I need to record messages and just feed them into the phone's audio input. The ghetto way would be to just record into my iphone, and put my iphone next to the other phone's mic... but I suspect that it makes me sound like a robocaller.

 

[Lutfij]

I think the option to record your voice and play it back is a good idea. Considering it's a flip phone and not something that's akin to a smart phone, your option is to record your voice and play it over from another source.

 

[Cowgoesmoo2]

I believe you could get something like a mixer and route audio though right