Is this a dialup PPP connection or PPPoE/PPPoA?
PPP configuration is in the FAQ, and there is an example configuration in
/etc/ppp/.
http://www.openbsd.org/faq/faq6.html#PPP
There is technically two implementations
pppd(8) and
ppp(8), you may find the syntax for ppp(8) with ppp.conf somewhat easier to understand.