The session border controller will be used instead of an ISDN gateway for outbound calls.
http://www.cisco.com/en/US/docs/ios/12_3/multiservice_ip2ip/guide/gw_ch2.html
Please see the section 'H.323-to-SIP IP-to-IP Gateway Connections' for your case.
http://www.cisco.com/en/US/products/sw/voicesw/ps5640/products_installation_and_configuration_guides_list.html
Restrictions for H.323-to-SIP Connections on an IP-to-IP Gateway
"Changing codecs during rotary dial peer selection is not supported.
"Delayed-media to slow-start signal interworking is not supported.
"Fast-start to delayed-media signal interworking is not supported.
"GSMEFR and GSMEFR are not supported.
"Media flow-around is not supported.
"Slow-start to early media signal interworking is not supported.
This should help alot.