It is possible to use your UK VoIP line abroad.
The call quality is determined by the broadband package you use out in Spain - it needs to support VoIP. Internet services can vary from country to country so be sure to check with the broadband provider in Spain that you have sufficient bandwidth and upload speed to accommodate both internet access and VoIP use.
So to summarise as long as you have a suitable broadband provider and router then yes you can take your VoIP phone equipment with you too.
How to set up?
1. Set up your phone equipment in exactly the same way as you do in the UK. If you are using a Gigaset Dect phone then plug the N300 base unit into the router. Plug the phone cradle into an electrical socket. Be sure to have a travel plug adapter to be able to plug into an electrical socket using a UK plug configuration.
2. You will find that your VoIP line call quality will be exactly the same as it is now.
3. The benefit of VoIP is that the cost of calls will be exactly the same cost as in the UK. You should have the same experience … nothing will change provided you have the right broadband plan to accommodate VoIP.