Many people use their PSTN lines for incoming calls and a VoIP line for outgoing calls as it saves them money. For such setups its recommended that you implement the following:
- Have your VoIP provider set your PSTN landline number as its default caller id. Now, when you dial out over your VoIP line it looks like your calling from your PSTN line.
- Get yourself an analogue telephone adapter (or DECT base) that has both an FXO and FXS port. This will allow you to use a POTS and VoIP line together in perfect harmony. (e.g. answer calls that come in on your pots line and dial out over VoIP)
As you have found out, hardware is limited. I think it's because old copper lines are being phased out and should be obsolete by 2025 and so sales of such hardware are low. Anyway, I recommend the following hardware:
- Grandstream HT813 ATA
- OBIHAI OBi212 (available on amazon.com only)
- Gigaset N300 IP DECT
- Gigaset N300A IP DECT (as above but has its own built in answering machine)
You can of course look on eBay for older hardware such as the Cisco SPA232D, Linksys SPA3000 or LINKSYS SPA3102. Be careful though, most are probably going to be cheap Chinese knock-offs (I wouldn't risk it myself as its a security risk IMO).