Get out of plat into diamond and communicate well to win as a team

I mainly play Sigma and Wrecking Ball with my Sigma being the better of the two. My season high this season is 2864 and I have been ranked platinum the last two seasons and the season before that gold. My main weakness as a Sigma is using my ultimate ability well. I believe this is because of a lack of communication with my teammates and not pairing it well with their ultimates, which as a team I would work on improving on this. My ball is above average and I am good at holding point, attacking the back line, and picking off weak enemies with him. Another hero I am learning is Roadhog, who I hope to have a decent grasp at soon.