Is internet provider generally the best choice for VoIP QoS?

How can I measure the lagginess of various 3rd-party VoIP providers? Has anyone compiled a list of IP addresses that I can ping, before choosing one?

I have some cool POTS equipment in my house, such as a payphone in my garage, with a big honking LED caller ID display mounted above it. I want whatever POTS-to-VoIP adapter I buy / get to handle the loads of it and about 2-3 other legacy telephony devices. Presently I am considering the Obi200, which has 1 RJ11 jack. Do you think I could not have done that, and have it work as expected? I also found that it was so easy to use both "internet" and "LAN" jacks, too, and I could not figure out why it would need both.