With PBX you can always have reachable phone lines. It allows you to manage incoming and outcoming calls at the same time as well as two or more calls at the same time. Incoming calls, directed to a main directory number, are sorted on the first line available connected through PBX to the other ones. When calling out, it is always the main PBX directory number to appear, even if the call is made from a secondary directory PBX number.
Cost: €2.44 per month for single numbering