Super Low Cost VoIP Provider?

I used to live in the UK. I still have my UK cell phone number (07) with a traditional provider.

I want to get rid of this and move to a VoIP solution. I very rarely use this number so would prefer to have as minimal cost as possible.

By coincidence, I have a UK landline number (01) with sipgate and that works just perfectly - I top up on their website occasionally and I get my messages in my inbox - great! Unfortunately they don't work with non-landline numbers.

Does anyone have any suggestions or recommendations?

Thank you!