actually what i mean to say is....

Check P2.0, P2.1, P2.2, P2.3 to detect off-hook of telephone sets which are connected through SLIC circuitary to controller..

off-hook give a HIGH signal while on-hook gives a low.

Now i have no specification at what rate should i check this off and On hook thing and i think there is no such specification as it depends where the exchange is installed and what is the communication traffic so designing the software for worst case where the users frequenctly picks up handsets and go on hook i have to design this monitoring in such a way so that all bits get equal timing....and none is missed.