The other option is to use something like this http://www.comx-computers.co.za/GS-HT-503-Grandstream-SIP-ATA-2x-10-100-LAN-Buy-p-55030.php The important thing here is it has an FXS port (which allows you to plugin an analogue phone) and an FXO port (which allows you to plug the landline into it so you can get incoming calls on the landline redirected to your VOIP phone.
In normal operation, you make outgoing calls using your normal phone, but it is redirected over a VOIP connection, so you get the cheap calls and incoming calls get passed through to your normal phones. But in the even of a power failure, you can still call out on your normal landline.
The other option is to get an FXS only gateway such as this http://www.comx-computers.co.za/GS-HT-802-Grandstream-SIP-ATA-1x-LAN-2x-FXS-Buy-p-153116.php which allows you to keep your existing phones but use a VOIP service for your incoming and outgoing calls. This is the option I chose because like most people these days, I hardly use the phone for voice calls and the gateway was much cheaper than buying a VOIP phone.
In normal operation, you make outgoing calls using your normal phone, but it is redirected over a VOIP connection, so you get the cheap calls and incoming calls get passed through to your normal phones. But in the even of a power failure, you can still call out on your normal landline.
The other option is to get an FXS only gateway such as this http://www.comx-computers.co.za/GS-HT-802-Grandstream-SIP-ATA-1x-LAN-2x-FXS-Buy-p-153116.php which allows you to keep your existing phones but use a VOIP service for your incoming and outgoing calls. This is the option I chose because like most people these days, I hardly use the phone for voice calls and the gateway was much cheaper than buying a VOIP phone.