HOLD SIP message problem

[farhad.jalali@gmail.com]

My T20 IP-Phone doesn't Ack to the OK message I passed to it while HOLD.

IP-Phone[192.168.1.90:5062] -> SIP Server[192.168.1.250:6000]

INVITE sip:09125170869@192.168.1.250:6000 SIP/2.0
Via: SIP/2.0/UDP 192.168.1.90:5062;branch=z9hG4bK1308374903
From: "a7bf0b2e-41d2-44bf-984e-48535a93009b@192.168.1.250" <sip:800@192.168.1.250>;tag=733854603
To: "Jalali.Mobile" <sip:09125170869@192.168.1.250>;tag=1929990922
Call-ID: a7bf0b2e-41d2-44bf-984e-48535a93009b@192.168.1.250
CSeq: 2 INVITE
Contact: <sip:800@192.168.1.90:5062>
Content-Type: application/sdp
Allow: INVITE, INFO, PRACK, ACK, BYE, CANCEL, OPTIONS, NOTIFY, REGISTER, SUBSCRIBE, REFER, PUBLISH, UPDATE, MESSAGE
Max-Forwards: 70
User-Agent: Yealink SIP-T20P 9.61.0.70
Allow-Events: talk,hold,conference,refer,check-sync
Content-Length: 194

v=0
o=- 20015 20016 IN IP4 192.168.1.90
s=SDP data
c=IN IP4 0.0.0.0
t=0 0
m=audio 11788 RTP/AVP 8 101
a=rtpmap:8 PCMA/8000
a=fmtp:101 0-15
a=rtpmap:101 telephone-event/8000
a=sendonly


SIP Server -> IP-Phone:

SIP/2.0 200 Ok
Via: SIP/2.0/UDP 192.168.1.90:5062;branch=z9hG4bK1308374903;rport=5062
From: "a7bf0b2e-41d2-44bf-984e-48535a93009b@192.168.1.250" <sip:800@192.168.1.250>;tag=733854603
To: "Jalali.Mobile" <sip:09125170869@192.168.1.250>;tag=1929990922
Call-ID: a7bf0b2e-41d2-44bf-984e-48535a93009b@192.168.1.250
CSeq: 2 INVITE
Contact: <sip:09125170869@192.168.1.250>
Content-Type: application/sdp
Allow: INVITE, INFO, PRACK, ACK, BYE, CANCEL, OPTIONS, NOTIFY, REGISTER, SUBSCRIBE, REFER, PUBLISH, UPDATE, MESSAGE

User-Agent: Yealink SIP-T20P 9.61.0.70
Allow-Events: talk,hold,conference,refer,check-sync
Content-Length: 211

v=0
o=- 3517046709 3517046709 IN IP4 192.168.1.250
s=SDP data
c=IN IP4 192.168.1.250
t=0 0
m=audio 16158 RTP/AVP 8 101
a=rtpmap:8 PCMA/8000
a=fmtp:101 0-15
a=rtpmap:101 telephone-event/8000

a=recvonly




Then IP-Phone doesn't send ACK. and because of that the UnHold doesn't work.