Aastra 53i showing SIP 400 "Bad Request"

An amusing gotcha that's taken me two days to pin down...

If you have an Aastra 53i up to firmware version 2.6.0.1008 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

Via: SIP/2.0/UDP

192.168.2.6:5060;branch=z9hG4bK7b016ea7;rport

From: "Unknown Name" <sip:John Smith@192.168.2.6>;tag=as269699d7

To: <sip:425.phone@192.168.2.192:5060;transport=udp>;tag=3007026722

Call-ID: 7e1d46dc3d5168b03720874818000e88@192.168.2.6

CSeq: 102 INVITE

Server: Aastra 53i/2.6.0.1008

Content-Type: text/plain

Content-Length: 23

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.