As ranked by ISP Planet, according to number of subscribers in Q2 2006.

Rank	ISP	Subs. (thousands)	Date & Source
1	Skype (paid VoIP only, customers worldwide, date of this number unknown)	1,800	[July 27, 2005] Press Release
1	Vonage	1,800	[as of June 30, 2006] Fact Sheet
1	Cox Digital Phone* (as of June 14, 2006)	1,800	[June 14, 2006] Press Release
4	Time Warner Digital Phone*	1,600	[August 2, 2006] 10-Q [.pdf]
5	CableVision (cable VoIP under the Optimum Voice brand name)	988	[September 21, 2006] Press Release
6	CallWave (free and paid VoIP, counting subscribers, not lines)	780	[May 12, 2006] SEC 10-Q
7	Comcast Digital Phone* (978,000 circuit switched customers not counted)	729	[July 28, 2006] SEC 10-Q
8	Charter* (cable VoIP)	258	[August 8, 2006] SEC 10-Q
9	Packet8 (a.k.a. 8x8) (counting lines, not subscribers)	151	[July 27 2006] Press Release
10	SunRocket	128	[September 27, 2006] SunRocket Public Relations
11	Insight Communications* (cable VoIP)	107	[August 9, 2006] SEC 10-Q
12	Mediacom* (cable VoIP)	66	[August 9, 2006] SEC 10-Q
13	Covad (business VoIP only, 1,343 VoIP business customers with a combined total of approximately 44,000 stations )	44	[August 3, 2006] SEC 10-Q
14	GCI (Alaksa only)	27	[August 9, 2006] SEC 10-Q
15	CBeyond (Alaksa only)	23	[August 14, 2006] SEC 10-Q
Note: data was unavailable for many VoIP service providers.
Note: services with an asterisk * are digital phone, not true VoIP