Sipdroid and asterisk

If you haven't found it yet, sipdroid is a small, reasonably well integrated sip client for android phones. If you install the version not available on market, you can also use it while using your 3g connection.

One drawback is that it doesn't neccessary play nice with Asterisk out of the box, these are the things I had to address to get it running smoothly (of course, YMMV).

You're most likely being NATed by your operator (if you want to be sure, you can install WhatIsMyIp and check), so

nat=yes

is pretty much a given. However, you do need

qualify=no

or your Asterisk will consider the phone unreachable.

Also, sipdroid currently only supports alaw, so you should make sure that no other codec will suddenly be required. If your asterisk is suitably located [low latency] and have ample cpu, you can use canreinvite=no for that, otherwise, you can try to restrict those you connect to so only alaw will be used, but that way may have many hidden surprises, especially if one of them is a PSTN operator.