Here's a workaround I used when I couldn't receive calls - most notably verification calls from my bank. It can be useful in the short term, although it's a bit longwinded to set up.
Requirements: your data works and you can enable "forward on unreachable" (you can test this with the code **62*PhoneNumber# where PhoneNumber is your desired number to forward to). Steps:
1. Sign up for a free UK landline number with Sipgate Basic
2. Download and install Zoiper and configure it as per your Sipgate details. You can try using your phone's system SIP account instead but on mine the quality of the call was not good.
3. Set up forwarding to your new Sipgate number using the code above.
Remember that for this to work, your data (3G) must be enabled or you need to be connected to a Wi-Fi network.