get a integer from char* ...?

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2

but getting the number part from the char* seems more tricky..

Using atoi() is tricky?

        char* msg = (char*)malloc(size+1);

Not a good idea. You know the maximum (reasonable) size of a packet. Allocate a fixed size array.

          value= String(msg[3])+String(msg[4])+String(msg[5]);

There is no excuse for converting each letter to a String. There is no excuse for using String at all.