Log file warnings from chan_sip in build_reply_digest

Using Asterisk 1.4.21.2 I am seeing pairs of warning logs of the form:

> asterisk[1432]: WARNING[1432]: chan_sip.c:11629 in  
> build_reply_digest: use realm [xxxxx] from peer [xxxxx][xxxxx]

These occur once an hour and the xxxxx matches the account name for my  
ITSP. My sip.conf setup for this account is a block copy from an older  
one that gave no warnings on my old Asterisk v1.2 setup:

> [general]
> register => xxxxx:secret <at> proxy.itsp.net
>
> [authentication]
> auth = xxxxx:secret <at> proxy.itsp.net
>
> [itsp]
> context=default
> type=friend
> username=xxxxx
> user=xxxxx
> host=proxy.itsp.net
> secret=secret
> insecure=very
> accountcode=ITSP
> amaflags=billing

Calls in and out work fine, but I would like to to have a warning free  
error log if possible.

It looks to me from