I've never used Asterisk, and perhaps I misunderstand what you're trying to do with SIP, but wouldn't the built-in VOIP client work for AOSP-Android? It may be

I've never used Asterisk, and perhaps I misunderstand what you're trying to do with SIP, but wouldn't the built-in VOIP client work for AOSP-Android? It may be hidden based on your carrier, but it's configurable if you remove your SIM, reboot, and set it up under Phone > Settings > Calls > Calling Accounts. Settings you enter will save if you reboot after as well.