I've found that unwanted dial-outs are often Windows boxes trying to do DNS lookups to find their domain controller. If you block outgoing netbios (a reasonable precaution), it won't stop them doing DNS lookups.

Do you have an internal DNS? If so, set it up as a primary for your internal domain (make one up, like .myhouse or something), and point all the other hosts at it. Set the Windows domain name for any Windows (& Windows-like) machine to be that domain (.myhouse or whatever).

If you don't have an internal DNS, it may be sensible to set one up. However you need a box that's always on. The Linux or Windows server boxes will do nicely provided they're always on and not the same box dual booting

If they Windows boxes do keep doing random DNS requests, the internal DNS should eventually cache them and decrease the number of unwanted dial-outs.