I've had similar experiences with silent teammates when maincalling on roamer. The most effective strategy I've found on my end is to ask for information. "Where is their demo?" "Where is their combo rolling out?" "Call focus fire." "Which Uber is better?" "Are we staying or leaving?" "What are Uber advantages?" Obviously this is a lot easier on pocket than roamer since you can see a lot more.
It's kind of a stopgap solution though - your teammates should be offering this information on their own, and asking them for it means that you will waste time and advantages will slip away.
The easiest solution is to just impress upon them the importance of communication and exactly why you feel frustrated as a decision maker without access to as much information as possible. Make sure you do so in a way that is not accusatory or they will get defensive and consequently become less malleable.
It helps to know why they don't do it. Often new players just don't know what to say. Sometimes they can't handle talking and DMing at the same time. Sometimes their mic is too quiet or their voice activation thresholds are too high. Sometimes they're shy and need time to adjust to a new group of people.
And sometimes they just don't talk.
It helps to know this because how much time it will take to get them to talk, if they ever will change, is a big deal to a team who plays a match every week. Is it worth the time investment to get them to eventually talk? Or should you cut them and look for replacements? And that changes with every team, especially when you have to weigh how important winning is to you as a team compared to playing with friends.