<--- SIP read from UDP:78.73.170.132:5060 --->

Immediatelly after that:
– Got SIP response 486 "Busy here" back from 78.73.170.26:5060
set_destination: Parsing <sip:123456710@sipdomain.com> for address/port to send to
set_destination: set destination to 78.73.170.26:5060
Transmitting (no NAT) to 78.73.170.26:5060:

78.73.170.26 is the IP of DNS A record for sipdomain.com. It is only mentioned in From/To fields. No idea why Asterisk is trying to find the IP in From/To fields. As far as I understand those fields are only informative. The destination IP is supposed to be the one from the previous INVITE or from Via fields.

Here the complete extract of the problematic SIP messages:
<--- SIP read from UDP:78.73.170.132:5060 --->
SIP/2.0 486 Busy here
Via: SIP/2.0/UDP 78.38.13.197:5060;branch=z9hG4bK2a032d14
From: "123456789" <sip:123456789@sipdomain.com>;tag=as4bbcf3c5
To: <sip:123456710@sipdomain.com>;tag=3a0313ac4fa7b5693eb322
Contact: sip:123456710@78.73.170.132:5060
Call-ID: 781e419e03536d8e0e37fe1e350a7ba5@sipdomain.com
CSeq: 102 INVITE
Server: Sip Proxy Server
Allow: ACK,BYE,CANCEL,INVITE,REGISTER,OPTIONS,INFO,MESSAGE
Content-Length: 0

set_destination: set destination to 78.73.170.26:5060
Reliably Transmitting (no NAT) to 78.73.170.26:5060:
CANCEL sip:123456710@sipdomain.com SIP/2.0
v: SIP/2.0/UDP 78.38.13.197:5060;branch=z9hG4bK0ea9d670
Max-Forwards: 70
f: "123456789" <sip:123456789@sipdomain.com>;tag=as379b0650
t: <sip:123456710@sipdomain.com>
i: 5aacbfe616438022583291ff53329eba@sipdomain.com
CSeq: 102 CANCEL
User-Agent: Sipura/SPA3000-3.1.3(GWa)
l: 0

<--- SIP read from UDP:78.73.170.132:5060 --->
SIP/2.0 486 Busy here
Via: SIP/2.0/UDP 78.38.13.197:5060;branch=z9hG4bK2a032d14
From: "123456789" <sip:123456789@sipdomain.com>;tag=as4bbcf3c5
To: <sip:123456710@sipdomain.com>;tag=3a0313ac4fa7b5693eb322
Contact: sip:123456710@78.73.170.132:5060
Call-ID: 781e419e03536d8e0e37fe1e350a7ba4@sipdomain.com
CSeq: 102 INVITE
Server: Sip Proxy Server
Allow: ACK,BYE,CANCEL,INVITE,REGISTER,OPTIONS,INFO,MESSAGE
Content-Length: 0

<------------->
— (10 headers 0 lines) —
– Got SIP response 486 "Busy here" back from 78.73.170.26:5060
set_destination: Parsing <sip:123456710@sipdomain.com> for address/port to send to
set_destination: set destination to 78.73.170.26:5060
Transmitting (no NAT) to 78.73.170.26:5060:
ACK sip:123456710@78.73.170.132:5060 SIP/2.0