The ones I’m aware of that people have used are in this thread: Text notifications for Cricket customers--SMS vs. MMS - #2 by wesochuck
Of the ones on that list, I would suggest that your most “hands off” option is probably going to be shoutbomb. Although I’ve not used them directly, I’ve known libraries who have. They are a very small operation but also have a very narrow focus which typically means a quicker turnaround time.
At my consortium we use MessageBee which works well for the level of customization and volume we need.