Drivers would be banned from dialing and even holding cellphones under a bill endorsed by the Legislature’s Joint Transportation Committee today. Proponents of the measure, which would make Massachusetts the 10th state to ban hand-held phone use while driving, say they want to finish the work they started with 2010’s Safe Driving Law. That law banned any form of cellphone use by drivers under 18 and outlawed texting behind the wheel for adults. But the difficulty of distinguishing between dialing and texting has made that ban hard to enforce.