strange how you couldn't get a PSTN trunk to appear on an IP handset. Must just be a programming issue. Should be no limitation on getting a PSTN trunk on an IP phone. Maybe not as good quality than ISDN for calls being made but certainly no limit to having a PSTN trunk appearing as a line key no

Yes - IP phones can have PSTN trunk appearances but had a lot of issues with call drop outs on IP phones on PSTN trunks