What are you exactly looking for? That an sms is sent when an issue is reported and/or as reminder?
That functionality does not exist yet to my knowledge but could be developed.

Sending a SMS has the need for a kind of gateway or alternatively a webservice.
Ozeki Ng is a commercial gateway whereas Kannel is an opensource alternative.
On the web you would find something like "http://www.mysmsaddress.com" which also provides a script which could be used to incorporate in any php application.
So question is what the best options should be for such a development? I am sure there are more alternatives (like slingshotSMS)

You can have it without internet but you need to have your own SMS server.
Also, there is no plugin yet !!

First you need to find someone who will create the SMS plugin for you.
In light of that development you will need to decide upon the sms server.
Since you do not want to use internet, you cannot use an online service but you need to implement your own. As indicated earlier, there are commercial and opensource variants.
In case you are allowed to go online, you can check with the service i mentioned earlier. On their website you will find samples on how this should be coded in php.