Co-workers expect me to answer the phone for them.

Co-workers expect me to answer the phone for them. I am a data entry and phone receptionist for a small retail store. I have been employed by them for three years. Part of my duties include bookkeeping and redirecting phone calls when the sales floor is busy. I was informed from day one that the salesmen are trained to quickly pick up their phone and ask the customers to hold and set the call in park until they can get with them. I am not a sales rep (I am bookkeeping) so I was trained to put calls on park only when the salesmen were slammed to help them out. I was only answering a call when they were absolutely busy and it wasn't a lot. I was able to focus on the computer aspect of my job, which I very much enjoy. And it kept my anxiety low because I have a block and stutter saying my company's name. Over time the salesmen got used to me helping them so much they started to let calls go (when they could easily answer them) knowing I would get it for them. My boss noticed this and has brought it to their attention several times in meetings letting them know I am **not** their secretary and that they need to answer their phones instead of expecting me to do it for them all the time. Sadly they always went back to expecting me to get their calls. I have counted how many times I answer the phone for them and it can be upwards to 130 times a day. I am basically getting every call and placing it on hold for them. This takes a lot of multi-tasking skill especially when I have to give return authorizations and someone is waiting on a return. I have to stop what I am doing to answer the phone for them. This would not be such a problem if I did not have a stutter. My stutter manifests as a block when I say my company's name and ask the caller to hold. This creates a very awkward silence and the customer thinks I am a sales rep and starts telling me their issue and I have to tell them to wait a second and then I place the call in park and page for someone. I also repetitively stammer on the first letter of my company name. Most of the time people cannot understand me and some have laughed and asked me if I forgot where I worked. Some just hang up or yell "HELLO?!" and breathe frustratingly into the phone. As you can imagine this takes a heavy toll on my self-esteem and causes my anxiety to skyrocket. I hate answering the phone because I know I am going to stutter and block. I worry we are losing business because people think they called the wrong place due to my speech impediment and hang up. I would not mind answering the phone, but I am doing it almost every time it rings for 4 different stores. I know the salesmen get busy, but they are not always busy to not be able to pick up their phone and quickly ask if the customer can hold. I have seen the salesmen chatting it up with each other at the counter while the phone rings and not even walk over to grab it. I have seen salesmen sitting right next to their phone as it rings and just look down at it, waiting for me to get it. My boss knows they do this, but nothing ever changes and they just keep expecting me to answer. I have been thinking about writing an email to everybody explaining it is difficult for me to perform my job duties while constantly having to answer the phone for everybody due to my stutter.