I have been using the SPA504G for about two years now with very satisfactory results. It's a 4-Line desktop IP-Phone, which makes it a great choice for registering with multiple accounts on one phone.
I wrote this blog article to try to make it as simple as possible to successfully configure the SPA504G with the Dell Voice Fongo SIP service:
http://voipdiy.blogspot.com/2012/02/del ... 4g-ip.html
So far, this phone is working great with Dell Voice Fongo for me. Hopefully, these configuration settings will work for others too.