I've also got mine registering to an asterisk box and I'm having no trouble with incoming or outgoing. I've used the default for keep-alive, didn't change anything. And, much to my delight, DTMF now works!

It sounds like since you have two accounts, maybe thats the problem with your Nokia device. Most firewalls do NAT, and if you've got two different machines on the network trying to talk SIP, that might be the issue.