so many challenges:

1.  Confidentiality of data: some participants think that their information might be disclosed to social media ( own observation). and this is very important in terms of Research ethic and good clinical practice remember!

2. Security: hackers can read sms, hence importance of  strong antivitus and hard copies.

3. Network problem, most of these data are connected to server which need strong network, in remote areas especially in Eastern part of Africa ( own experience) it is very difficult to have full access of internet. This might prevent from reaching some participants

4. An other thing  to consider, do all participants have mobile phones? if yes, are their mobile phones original or fake? do they have  airtime for keeping in touch with the clinician?

5. At what time do you send sms to participants? in a country like India where culture  plays a big part it is not good to send sms at midnight.

6. Education problem: in order to implement that program the whole family should get a training otherwise sms can be misinterpreted and cause chaos in families: ( I have seen this)

This is a very important question and very nice points from Shabani.

I would add that in Mozambique we managed to see a small improvement of adherence to medicine and a slight 12 months retention among patients newly ART initiated on urban sites. However, somewhere at month 9-11 month the effect vanish very rapidly. It seems people got tired of receiving such messages and we received complains about SMS.

As Shabani said, besides the SMS content, things like electricity (is the health facility with power to feed the SMS system?) and network outages are critical to send SMS; Local expertise to keep the system running; Literacy (we sent text SMS), played key role.

I agree with all what Mueller and Augusto mentioned.  They have mentioned all the technical challenges that limit the practicality of the intervention.  

Challenges related to the design of the intervention: Mobile health interventions are mostly behavioural science interventions. As it is not like biochemical drugs that enter into the body and bring a change immediately. SMS, App, web-based interventions, etc are behavioural change actors. Their effectiveness particularly long-term effectiveness is complicated. Most of the mobile-based interventions studies are pilot projects that last 4 weeks to 12 months. How long does it take to change the target behaviour is not well known? or it is mostly affected by individual patient behaviour.  So Tailoring the intervention is a key determinant of success. However, tailoring the intervention limits the number of participants you can handle as well as the data collection and analysis. 

Another issue is "Is the intervention based on behavioural science or health education models/theories?" As I mentioned above mobile health interventions are mostly behavioural in action. However, most of the mHealth interventions never consider the major scientific guidance to bring the change. eHealth intervention is highly undertheorized.