T58W - how to configure several SIP accounts

[Yisroel_MongoTEL]

I do not think that's possible, all extensions registered will remain registered regardless of the account selected as the active one (which will be used as default for placing calls.)

What you might want to take a look at is the Hot Desking which can allow you to easily swap the account on the phone. But this generally also needs to be supported by the VoIP system you're using.