Do you have RFC2833 enabled on the provider end? In your INVITE you should see "a=fmtp:101 0-15" coming from the IP Office. This tells the provider that the IP Office supports DTMF events 0-9, *, # and A-D.
The provider should send back an fmtp message with their supported DTMF events. Here is an example:
IPO to Provider:
835437366mS SIP Trunk: 20:Tx
INVITE sip:1xxxxxxxxxx@did.voip.les.net SIP/2.0
Via: SIP/2.0/UDP xx.xx.xx.xx:5060;rport;branch=z9hG4bKee09556c185a0975b244acf5d9e57551
From: "KyleHolladay" <sip:xxxxxxxxxx@mysipline.net>;tag=69bd8f80bfd83b8f
To: <sip:1xxxxxxxxxx@mysipline.net>
Call-ID: 8c4d52ae1c30ef975c1ddd9808f48269@xx.xx.xx.xx
CSeq: 1682787794 INVITE
Contact: "KyleHolladay" <sip

rimeServicesLLC@xx.xx.xx.xx:5060;transport=udp>
Max-Forwards: 70
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, INFO
Content-Type: application/sdp
Content-Length: 277
v=0
o=UserA 1962482231 1291585475 IN IP4 71.39.23.171
s=Session SDP
c=IN IP4 xx.xx.xx.xx
t=0 0
m=audio 49154 RTP/AVP 18 0 8 101
a=rtpmap:18 G729/8000
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=fmtp:18 annexb = no
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
Provider to IPO
835442048mS SIP Rx: UDP xx.xx.xx.xx:5060 -> xx.xx.xx.xx:5060
SIP/2.0 183 Session Progress
Via: SIP/2.0/UDP xx.xx.xx.xx:5060;branch=z9hG4bKee09556c185a0975b244acf5d9e57551;received=xx.xx.xx.xx;rport=5060
From: "KyleHolladay" <sip:xxxxxxxxxx@mysipline.com>;tag=69bd8f80bfd83b8f
To: <sip:13033502352@did.voip.les.net>;tag=as0768fb8e
Call-ID: 8c4d52ae1c30ef975c1ddd9808f48269@xx.xx.xx.xx
CSeq: 1682787794 INVITE
User-Agent: SIP.VoIP
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Contact: <sip:1xxxxxxxxxx@xx.xx.xx.xx>
Content-Type: application/sdp
Content-Length: 239
v=0
o=root 16383 16383 IN IP4 xx.xx.xx.xx
s=session
c=IN IP4 xx.xx.xx.xx
t=0 0
m=audio 16498 RTP/AVP 18 101
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=no
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=silenceSupp

ff - - - -
Kyle Holladay
ACA-I, ACA Call Center, ACS-I, ACS-M, TIA-CTP, MCP/MCTS Exchange 2007
ACE Implement: IP Office
"Thinking is the hardest work there is, which is the probable reason why so few engage in it." - Henry Ford