Does SMS format depend on the sender's mobile?
If you're not trying to capture the mobile number or whatever SMS text was sent, but only react to specific SMS messages, then don't try to parse the whole message, but design the protocol between any arbitrary phone and your Arduino to be detectable independent of everything else, e.g.:

#L 5 on        ; set an LED state&lt;br&gt;
#M 3 137       ; set a motor speed&lt;br&gt;