For the first problem I do not have ht503 but i make extension on my elastix pbx and setup icalldroid like trunk to use this extension. Its written REGISTERED with green letters. The difference is that i didn't write nothing in from-domain field. The second problem is little bit difficult to fix because like you say it not chronic. Some times hang-up some times not. I think that you need to find dial tone settings from your PSTN and setup fxo port on ht503.
This maybe can help:
And in your pictures i could not see codec priority on ht503 but if it working than its ok. Any way can be possible that ht503 have some bug and need firmware update?