If its not single strand, its probably not telephone cable. If its flat and multi- core, it could be cordage, which is multi stranded, if its circular cross section and stranded, its likely to be alarm cable. A lot of telephone connections are insulation displacement types (like modified scotchlock), and these dont work well with strands, since the stranding is flexible enough to twist out of the way thus avoiding connection. Strands work better with screw terminals than with IDC.
Single strand only becomes a problem when used improperly - unless its a very short length, it should be supported close to the electrical connection, and of course, the boat has been built so that there isn't a lot of vibration, hasn't it?