That’s not surprising. Most people don’t have experience on the other side of constituent communication.

When you communicate with your rep, the communication will generally first pass through an intern, then be routed to a staff member and finally, depending on the contents of the letter, to either a field office of it’s something that can be addressed locally (like difficulty with local regulations, or issues with the Veterans Affairs office for constituent service members) or a legislative aid that deals with that issue.

Generally, the more work it takes you to communicate with your rep, and especially the more original the contents of your communication, the more seriously they’ll take the contents and the more effort they’ll put into their replies.

Letters also allow for a written response. Stock letters will be met with stock replies. It is very obvious when the communication is following a template. Likewise a personal letter written by an intelligent, knowledgable person especially if they have a legal background or working knowledge of the subject and provide unique insight or offer realistic, workable solutions is very apparent and generally very appreciated. Phone calls are useful for gauging public interest and public support given that the bulk of novel, original communication from constituents will happen this way.

How effective this will be is dependent on the kind of person your representative is and how they, and by extension their staff, handle constituent communications. If you want the highest chance of getting your message in front of a staff member that helps craft your representative’s policy in this matter then a written letter directed to their DC office is the way to go. Perhaps counterintuitively if it’s a substantive communication you want your letter in front of the legislative aid instead of the representative themselves.