Alex Balashov via sr-users writes:
SIP Outbound (RFC 5626) is, I think, the officially
sanctioned remedy,
but, while Kamailio supports it, most customer equipment in the field
to date, as far as I know, does not.
Yes, that is the best solution. SIP UA registers via two outbound
proxies and is thus able to receive INVITEs from both. There are SIP
UAs that support it.
-- Juha