Busy signal when answering PSTN to VOIP calls

I am trying to setup a Speedtouch 716WL with a Tesco Internet Phone account.

Almost everything is working OK: i.e.
1. Landline calls to/from my landline number are successful.
2. Tesco VOIP calls to/from Tesco Internet Phone are successful.
3. Tesco VOIP calls to landline numbers are successful.
4. But Landline calls to my Tesco Internet Phone result in the phone ringing but when I pick-up, I only hear a 'busy' tone.

Is there a setting that need to be made to receive landline calls to my Tesco Voip number?

FWIW, Landline calls to my Tesco Voip number that I accept via my PC don't have this problem.

from the DOS prompt


telnet 192.168.1.254
Username:Administrator
Password: (enter password set for router, if no then leave it blank)

try the below three commands

voice country config country=uk

voice config digitrelay=inband

saveall