Voipfone supports open networks and will partner (it's called peering) with any VoIP provider who wishes to. Just like calls between Voipfone customers, call between partner networks are totally free.
http://www.voipfone.co.uk/Peering.php
Voipfone Peering
#1Regards,
Voipfone Customer Services
iNet Telecoms Ltd (Voipfone)
Sovereign House
227 Marsh Wall
London
E14 9SD
United Kingdom
Registered number: 05168033
Vat Registration Number 858850966
Telephone: 020 7043 5555
Fax: 020 7043 5556
Web: http://www.voipfone.co.uk
Blog: http://www.voipfoneblog.co.uk
Forum: http://www.voipfoneuserforum.co.uk
Twitter: http://www.twitter.com/voipfone
Voipfone Customer Services
iNet Telecoms Ltd (Voipfone)
Sovereign House
227 Marsh Wall
London
E14 9SD
United Kingdom
Registered number: 05168033
Vat Registration Number 858850966
Telephone: 020 7043 5555
Fax: 020 7043 5556
Web: http://www.voipfone.co.uk
Blog: http://www.voipfoneblog.co.uk
Forum: http://www.voipfoneuserforum.co.uk
Twitter: http://www.twitter.com/voipfone