Use ～を通じて when stating the medium, channel, or intermediary by which something is done — an agency, a contact person, email, a survey — or, with a time span, to mean "throughout." It is the written/formal counterpart of で or 〜で連絡する, common in reports, contracts, and email. Near-synonym ～を通して is slightly more agentive (you deliberately used the route); ～を通じて more often just states the channel and is the safer choice in documents.