The Draytek IP-PBX and Draytek BX2000 were Draytek's flagship router of the day, the Draytek 2830 and Draytek 2860 respectively with a phone system board added. These routers have continued to evolve and now the Draytek 2865 and Draytek 2866 with Gigabyte throughput on the WAN can provide the router / firewall element, there are a number of options for the phone system.
If you only need less than 8 extensions then add the Gigaset or Yealink with DECT handsets registered to them may meet your requirement. If you need the handsets to work further than the coverage of a single DECT base then you either need a "Cloud" hosted system or a VoIP system with controller hosted on your own VM (Virtual Machine running on your own server) or on its own dedicate box.
Our SpliceCom SV1000 will support anything from a couple of phones to many thousands, just call us for a chat and we can advice you on the exact configuration that is best for your needs.