cast a spell and your cat speaks like Salem is Sabrina [showing my age lol] doesn't exist. magick works with energy and forces, and it cannot go against nature. cats don't posses the part of the brain which is necessary for speech therefore the spell would not work. granted, you can communicate with your cat and connect with your cat, even understand it through body language and tone of its voice.
telepathy i'm not sure, thats a bit of a grey area, and since i haven't really looked into it, i can't say yes or no.
regarding your cat as your familiar, how do you know? a familiar is an animal, true, but you have a close spiritual bond with this animal, you can't cast a spell, nor point at a pet and say 'you're my familiar.' i had one when i was a child, its a very unique bond. i have had animals in my life since i was born, and this one we had such a connection, he even stayed with me after he died, even after moving to a different province. [i know not everyone believes in spirits] try meditating with/near your cat, see if you sense anything. while a familiar isn't always calm, there is a spiritual bond. i know people whose familiar would help them with their craft. [my friend had one who would sit outside the circle waiting for it to be open. my cat has to be locked in a room lol]