ttyp0 permissions

[richard]

I'm running potato and modemu has developed a problem.  I start it with
modemu -e "AT%B0=1%B1=1&W" -c "minicom -o -p tty%s" .
The tty%s always translates to ttyp0.  Recently the permission
on ttyp0 has become
crw-------   1 root     tty        3,   0 Jul 23 21:37 /dev/ttyp0
I hit it with chmod go+rw and it works for a while, then gets
changed back.  I have no idea what's causing this or where to
look for help.