SIP TO: is needed to change a user domain name

Hi Gurus.

I faced configuration issue.
Please, let me direction which I can digg to.

My asterisk gets incomming SIP call from soft phone.
Asterisk knows a calls of this subscriber have to be forvarded to soft switch accorging with configuration:

:extention.conf
[to-soft-switch-phone]
exten => t213,1,Dial(SIP/${EXTEN}@soft-switch-out,60,rt)

:sip.conf
[soft-switch-out]
type=peer
context=to-main
secret=t138
defaultuser=t138 ; Authentication user for outbound proxies
fromuser=t138
fromdomain=easyfon.com
host=ph.domain.com
insecure=invite,port

Asterisk does it.
But on soft switch side I collect traces and see
TO: t213@ph.domain.com blah blah blah

The soft switch rejects such INVITE just because it understands domain ‘easyfon.com’ in TO field only.

So … I need to change SIP address in the TO: field somehow.

Is it possible or not?
If yes, how? Do I need to develop some extention or C-programming is needed … or … I don’t know …

Anyway, Any advices would be very appreciated.

P.S. Field FROM: is correct - t138@easyfon.com.

Regards.
Ru-ki-ra.