There are some great answers here, I just wanted to roll them up together.
Universities are going more modern, so I don't think it's unreasonable that they ask professors to have a mobile phone type way of communicating with students as an alternative to traditional office hours. However, I think they are lagging in helping professors set boundaries.
Here's an idea stemming from some of the great answers here:
1 - Get a work-specific mobile number
Some folks get a second line on their existing phone through their service (costs money). Google voice (http://www.google.com/googlevoice/about.html) is a free option. I suspect there are several other services, with varying prices. The limitations are somewhat connected to your region.
- VOIP based services that let you respond to SMS through a web app (google for "temporary phone number for SMS" got me tons of options)
- second phone number linked to current phone - "two mobile numbers one phone" got me many solutions for this.
- second, cheap mobile phone that you can burn when class is over
Things you'll want to consider:
- ease of destruction - how do you turn it off?
- access - does it route to your phone? Do you login into something?
- does it to voicemail and SMS? Do you want it to?
- will the school pay (if not, it may be a good tax write-off)
2 - Set Boundaries
First day of class should always include the rules and boundaries. Along with the grading policy, the basic curriculum, and the rules of in-class behavior; it's always the case that you need to set terms and conditions for getting help from the professor inside and outside of class. Taking the analog of pre-mobile communication, it's more than fair to include:
- your availability - for example, will you answer questions w/in 30 minutes from 9-5, M-F? Do you have some nights when you are free to answer questions? What's the turnaround time on the weekend? I know we live in a world of instantaneous answers, but it's important to be clear on what you can actually do. Better that they know this than that they text you 20 minutes before class with a problem on the homework assignment while you're driving to class yourself and won't answer the text.
- consideration for your student's availability - When setting up guidance, keep in mind that your students may live a different lifestyle than you do - and try to fit in at least some times that would reasonably work for them. For example, high school students are presumably doing homework after school and on weekends. College students have classes at various times in the day and may do homework in between classes. Night school students probably have a day job and spend a great deal of time doing homework on weekends. Pick availability that works for both you and the students and you'll get less complaints.
- a definition of what "reasonable help" is. You can't make 200 different sample questions. If that's a high demand item, and it's reasonable, offer some sources for sample questions, and then say this isn't fair to text you about. Be clear about when you can handle a text vs. when you want a phone call vs. when you want the person to book time from you. Keep in mind that use of texting varies with the person/generation/culture so your students may have a different definition of how long and complex a text based chat can be before it becomes infuriating to you. For example, when I manage groups of folks who are more than 10 years younger than I, I say "look, I can text but if I have to send you more than 3 quick messages to answer your issue, I'm going to call you."
3 - Stick to it
Enforce the rules. Don't respond outside your promised hours. Do respond as you said you would. Hold students accountable.
Mobile phones actually help with this - on many phones, you can set access control by person - so you can make sure that people on your known contacts list (friends and family), can call you night and day while the phone denies interruptions after certain hours for unknown callers/texters. You can also give unknown numbers (or known students) a special ring tone so you know what group is contacting you.
Give feedback and share generally useful stuff widely. If a student is monopolizing your time to the point where you are helping unreasonably - tell the student. This can happen even without texts, but texts can mean it's more intrusive, and they lower the barrier making it easier to annoy other people without much effort. And - if you get 3 texts asking for the same basic thing, cut it off at the pass, and give everyone the info -- or tell everyone that you won't be answering that question, since if 3 people thought it up, you can bet that several others will get there eventually -- they just haven't texted you yet. ;)
Definitely get a class text list set up (maybe let students opt out) so you can broadcast an answer easily.