I've recently switched from SipDroid to CSipSimple (https://market.android.com/details?id=com.csipsimple&fea...) - which in my opinion works much better. The main difference is that SipDroid implements almost everything in Java code, while CSipSimple is a wrapper around PjSip (which is a pretty mature SIP library).

So if this doesn't work in SipDroid it may be possible that it works with CSipSimple.