Assuming they don’t do a voice vote (PDF).

Another example of more subtle forms of exclusion afforded by anonymity can be found in the practice of voice vote in the US Congress. While the roll call voting record, which identifies the voting behavior of each Congress member individually, has established itself as the dominant practice in modern congressional procedures, the default version still practiced today is the voice vote. Here yeas and nays are each expressed verbally and collectively so that the individual vote of the respective Congress member remains unknown to the public. The chair then gauges the majority and decides the vote. This procedure, which is the original practice of the US Congress, undermines transparency and parliamentary accountability. Obscuring voting behavior effectively shuts the public out and metaphorically speaking closes the doors of parliament to public scrutiny. Lynch and Madonna (2103) find that elections incentivize Congress members to request a recorded roll call vote. Thus, while the competition for seats enhances transparency, voice voting is still a common practice when members of Congress want to conceal their voting behavior on controversial issues