I still use it to make international calls to family (it's my only use case for it). Does anyone know of a good replacement (preferably FOSS) that allows calling on landlines (yup, old people still use them)?
You can do this using JMP Chat, which bridges phone numbers to XMPP. Unfortunately, it's USA and Canada only, for now.
Alternatively, you can use a "SIP Trunk Provider". For instance, in the UK, Andrews and Arnold offers this service. You can then connect any compatible SIP client (e.g. Gnome Calls).