I would look into having your employer provide a Satellite phone such as This

Most Motorola phones have an external antenna port... certainly my e815 and hubby's v710 do (Verizon) - as for a coverage locator - I was recommended to a universal kinda site that turned out to be terribly inaccurate for my rural area - the major carriers will each have a coverage map on their site that you can reference, but geography will always be in the way creating dead spots and reflected signals where there are hills and valleys.

I study the behavior and ecology timber wolves in northern Wisconsin. Being a rural area, we have a lot of 'dead spots'. My cell phone is a vital safety link when I am out in the field, several miles away from help. For the past several years, I have used a Nokia 5165 connected to an external antenna booster. This has worked very well for me. Now I am in the market for a new phone and a new provider. Because of our poor coverage, I want to continue to use the booster. Can anyone suggest a site where I can find a listing of cell phones that heve an external antenna connector? Is there a site where I can compare coverage in my area for various providers?