If you have an Aastra 53i up to firmware version 126.96.36.1998 connecting to an Asterisk 1.4.x box do _not_ use the "fromuser=" directive in the SIP.CONF file associated with that phone. The phone won't accept calls from Asterisk any more, returning error code 400 "Bad Request".
<--- SIP read from 192.168.2.192:5060 --->
SIP/2.0 400 Bad Request
From: "Unknown Name" <sip:John Smith@192.168.2.6>;tag=as269699d7
CSeq: 102 INVITE
Server: Aastra 53i/188.8.131.528
Invalid header(s): From
... as soon as I removed the "fromuser=" lines from the SIP.CONF file and did a SIP RELOAD at the console, the phones were immediately able to take calls as expected.
Shout-out to Aastra T2 tech support who put me on the right path.