DTMF problem over IAX2 trunk

Hello Experts,

We have Asterisk 1.4.33 with fallowing onfiguration:

PSTN—Asterisk—IAX2 Trunk (GSM)-----ASterisk----IPPHONES

When we call PSTN number IVR just starts playing but when we try to type extension we have That’s not a valid extension …

Trunk info:

context=from-internal
disallow=all
allow=gsm
host=SomeIP
qualify=yes
secret=somepassword
type=peer
username=some user

IAX jitterbuffer off did not help.

Please advice what can also be done…

Thank You