1. Of an entity or process, to be unable to proceed any further and to reach the greatest amount, extent, or degree that is possible or allowed. English = (be) limited.
2. Of telecommunications (phones, TTYs, etc.), to be already in use, so you cannot communicate with the person you are trying to contact. English = (be) busy, (be) engaged.
3. Idiomatic English = unable to get through, unable to be put through.