The way to tell if an incoming call is an SMS call or a voice call when receiving incoming SMS on an Analog or ISDN line in Asterisk is by reading the callerid.
Normally there is only one Short Message Service Center (SMS Center) which can send you SMS on a fixed line. By looking at its callerId you can know if it is an SMS call.
In the mobile world, nevertheless, we can not consider an SMS as a call, and the cellphone won’t send a SMS directly to the landline phone. What it will do is to hand the SMS to the carrier used by the mobile SMSC, which then will hand it over the SMSC used by the landline carrier.