Are you refering to a Video Call from the Phone Keypad to another Mobile Number? If so, you may need to get customer service to enable it on your account. And watch out for the charges, there not normally included in plans, and can be $1 per min. As this is an old standard by today (64k circuit switched), have you considering using something like Skype or Facebook Messanger to do video calls? There much better qualtiy and free over Wifi, and and on Mobile only use some of your data, rather then an additional per minute rate.
There are lots of clients, Google has its own Duo, depends if your calling from Android to Android or Android to iPhone. As there all free, give each one a go and find the one that works for you. Circuit Switch calls are starting to be removed by carriers, and not used as the quality now days at 64k is terrible on a large screen, and even removed off some devices like iPhone etc. Hope you find something that works for you and get a great experience!