sorry if this is completely the wrong forum, but I figured someone here would know.

If I've got 3 BT business telephone lines (connected to a PBX system) can I make it so that someone calls our main line phone number and its engaged, the call will be put through on the second or third line? Is this a service that BT can provide? I don't want an 0845 number because that is more expensive from some places, and doesn't work internationally from everywhere. I know that using featureline will do it, is that the only way?