[Nexus S General] Chahk:
I'd like to know this as well. I tried setting up an account in Settings -> Call settings -> Accounts:

Username: +1XXXYYYYYYY (where XXXYYYYYYY is my Google Voice number with area code)

Password: my 4-digit access code

Server: sip.voice.google.com

Set as primary account: check

But when I tap "Receive incoming calls" it fails with the following logcat (my actual GV number masked with "1XXXYYYYYYY"):

D/SipSession( 394): start stack for sip:+1XXXYYYYYYY@sip.voice.google.com
W/SipSession( 394): command error: java.util.EventObject[source=Deregister]
W/SipSession( 394): java.util.regex.PatternSyntaxException: Syntax error U_REGEX_RULE_SYNTAX near index 1:
W/SipSession( 394): +1XXXYYYYYYY@
W/SipSession( 394): ^
W/SipSession( 394): at java.util.regex.Pattern.compileImpl(Native Method)
W/SipSession( 394): at java.util.regex.Pattern.compile(Pattern.java:400)
W/SipSession( 394): at java.util.regex.Pattern.<init>(Pattern.java:383)
W/SipSession( 394): at java.util.regex.Pattern.compile(Pattern.java:374)
W/SipSession( 394): at java.lang.String.replaceFirst(String.java:1982)
W/SipSession( 394): at com.android.server.sip.SipHelper.createRequest(SipHelper.java:225)
W/SipSession( 394): at com.android.server.sip.SipHelper.sendRegister(SipHelper.java:202)
W/SipSession( 394): at com.android.server.sip.SipSessionGroup$SipSessionImpl.readyForCall(SipSessionGroup.java:976)
W/SipSession( 394): at com.android.server.sip.SipSessionGroup$SipSessionImpl.process(SipSessionGroup.java:641)
W/SipSession( 394): at com.android.server.sip.SipSessionGroup$SipSessionImpl.processCommand(SipSessionGroup.java:593)
W/SipSession( 394): at com.android.server.sip.SipSessionGroup$SipSessionImpl.access$1500(SipSessionGroup.java:387)
W/SipSession( 394): at com.android.server.sip.SipSessionGroup$SipSessionImpl$1.run(SipSessionGroup.java:516)
W/SipSession( 394): at java.lang.Thread.run(Thread.java:1019)
V/SipSettings( 394): profile uid is 1001 isPrimary:true registration:Account registration failed Primary:sip:+1XXXYYYYYYY@sip.voice.google.com status:Account registration failed

Looks like it doesn't like the "+" in the account name. Tried it without the "+" and am getting a timeout error (also masking my phone's IP address with "ZZ.ZZZ.ZZZ.ZZ"):

V/SipSettings( 394): New Profile Name:1XXXYYYYYYY@sip.voice.google.com
V/SipSettings( 394): addPreferenceFor profile urisip:1XXXYYYYYYY@sip.voice.google.com
V/SipSettings( 394): profile uid is 1001 isPrimary:true registration:Checking status... Primary:sip:1XXXYYYYYYY@sip.voice.google.com status:Checking status...
D/dalvikvm( 394): GetFieldID: unable to find field Ljava/util/HashMap;.loadFactor:F
D/dalvikvm( 394): GetFieldID: unable to find field Ljava/util/HashMap;.loadFactor:F
V/SipSettings( 394): addPreferenceFor profile urisip:1XXXYYYYYYY@sip.voice.google.com
V/SipSettings( 394): profile uid is 1001 isPrimary:true registration:Checking status... Primary:sip:1XXXYYYYYYY@sip.voice.google.com status:Checking status...
V/SipSettings( 394): profile uid is 1001 isPrimary:true registration:Registering... Primary:sip:1XXXYYYYYYY@sip.voice.google.com status:Registering...
D/SipSession( 394): +++ add a session with key: 'c3adb34eff8ddbd083e006cbe188944d@ZZ.ZZZ.ZZZ.ZZ'
D/SipSession( 394): c3adb34eff8ddbd083e006cbe188944d@ZZ.ZZZ.ZZZ.ZZ: @406219f0:DEREGISTERING
D/SipSession( 394): remove session @406219f0:DEREGISTERING @key 'c3adb34eff8ddbd083e006cbe188944d@ZZ.ZZZ.ZZZ.ZZ'
D/SipSession( 394): close stack for sip:1XXXYYYYYYY@sip.voice.google.com
V/SipSettings( 394): profile uid is 1001 isPrimary:true registration:Account registration failed Primary:sip:1XXXYYYYYYY@sip.voice.google.com status:Account registration failed
D/SipSession( 394): start stack for sip:1XXXYYYYYYY@sip.voice.google.com
D/SipSession( 394): +++ add a session with key: '6518678e486e1976b9121e3e0b8cd3f3@ZZ.ZZZ.ZZZ.ZZ'
D/SipSession( 394): 6518678e486e1976b9121e3e0b8cd3f3@ZZ.ZZZ.ZZZ.ZZ: @407613c8:DEREGISTERING
V/SipSettings( 394): profile uid is 1001 isPrimary:true registration:Registering... Primary:sip:1XXXYYYYYYY@sip.voice.google.com status:Registering...
D/dalvikvm( 2904): GC_EXPLICIT freed 35K, 53% free 2737K/5703K, external 0K/512K, paused 54ms
D/dalvikvm( 2282): GC_EXPLICIT freed 3K, 57% free 3226K/7367K, external 5242K/6453K, paused 70ms
D/dalvikvm( 2507): GC_EXPLICIT freed 5K, 50% free 3475K/6855K, external 0K/0K, paused 59ms
D/dalvikvm( 1769): GC_EXPLICIT freed 178K, 50% free 3470K/6855K, external 0K/0K, paused 73ms
D/SipSession( 394): session key from event: 6518678e486e1976b9121e3e0b8cd3f3@ZZ.ZZZ.ZZZ.ZZ
D/SipSession( 394): active sessions:
D/SipSession( 394): ...6518678e486e1976b9121e3e0b8cd3f3@ZZ.ZZZ.ZZZ.ZZ: @407613c8:DEREGISTERING
D/SipSession( 394): transaction terminated: req=REGISTER,2463,s=TRYING,ds=-,
D/SipSession( 394): processing Timeout...
D/SipSession( 394): remove session @407613c8:DEREGISTERING @key '6518678e486e1976b9121e3e0b8cd3f3@ZZ.ZZZ.ZZZ.ZZ'
V/SipSettings( 394): profile uid is 1001 isPrimary:true registration:Account registration failed: (registration timed out); will try later Primary:sip:1XXXYYYYYYY@sip.voice.google.com status:Account registration failed: (registration timed out); will try
later