JAIN-SIP is enough for anything. If you find an unsupported use-case let me know and I will fix it.
The SIP stack inside android itself is JAIN-SIP (wrapped in different APIs). JAIN-SIP is at the core of almost all open-source Java SIP phones and servers - Jitsi, Mobicents, JAIN-SLEE uses JAIN-SIP by spec and may be 100s of other projects. No matter what you do you will probably end up using JAIN-SIP behind the scenes anyway :)