Must I check last box SIP Register (if unsure set to NO) to YES?
 

If I understand correct,

1) I set SIP Register to YES e.g. for provider Free world dialup with sip gate fwd.pulver.com

2) I am logged on only on sip number by voxalot.com

3) When I will try to call my account sip:XXXXXX@fwd.pulver.com what should happen? Ringing on voxalot or number is offline? Am I also automatically registered by provider with SIP Register to YES? That is very important when I have by this provider also PSTN number assigned.

Yes, for inbound calling It is necessary
 

It take about up to 30 minutes to appearing status registered on web page, but it works soon in real. I have read status is every 5 minutes updated.

So for my problem I register my provider in provider list and set Status Registered to YES. Than I set on my IP phone voxalot.com account and I can income calls on PSTN number assigned to old provider in provider list. And more I can call out via ENUM :)