What is the best method for texting a large number of mobile phones users at once with my pc?

my company has been using MS outlook to send text messages to our employees about priority situations for the better part of the past 5 years. While we have finally figured out all the technical ins and outs that make this system work properly and efficiently (i.e. distribution list size, provider email addresses, etc) we have reached a point where the number of lists we have (11 dist. lists) require too much time to compose and send messages to, to make this a viable solution for us anymore. Our budget for this project is not large so the ideal solution would be a computer software program that would be a one time purchase and does not require monthly fees of any variety to maintain and utilize. I realize that this is a hefty request but any suggestions anyone could offer would be greatly appreciated. Thanks in advance!