Once you ‘port’ or transfer your number to Truespeed, your current copper phone line is deactivated by your current provider. This means any services that used the old phone line won’t be available any more.
You won’t get charged for having two phone lines for one property, but it’s a good idea to let your current broadband/call provider know that you’re leaving them, as there may be cancellation fees if you’re still in contract, but please do not cancel your current contract until your Truespeed service is live.