>> 3.mobile phone sends sms message in pdu format to pic.
>> 4.pic recognises message certain bytes
> >5.pic switches on an output pin (to do anything)
>
>Probably a bit easier using a PIC with an USART.
>But maybe you need the small size of the 12F ?

I would agree, in which case a 16F627 or 628 would be better, same 
instruction set, but with slightly more pins and a hardware UART. 

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