How can i trust a man who is a habitual liar?

checking on a problem with our cell phone acc. i found my man has been calling a female co-worker almost 14 -20 times a day for months he says there was nothing to it .just talking but he had changed dramatically at home even tho i was caring for my son who has cancer. he became nasty and bored and couldn't wait to go back to work. he lied about it when confronted. i continued to catch him in lies the phone number he called is no longer on the bill but i blocked it.he said he blocked it and i never corrected him just found it interesting that he defends her and himself whenever i sound off about it