DTMF issue after hold the call

Hai ,
I logged as a agent in asterisk . Then put a channel in the hold and released the hold . after some time i put a call in the hold and released the hold . After that , whatever the DTMF which i am pressing , asterisk doesn’t recognize . In asterisk console I put the command to trace the DTMF .
core set debug channel SIP/1001-
In asterisk console , asterisk keeps on printing the below line

<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000000]

Hi ,
The agent logged into the queue . Then he pressing maximum 2 digits as DTMF . Then asterisk keeps on printing the in console .
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]
<< [ TYPE: Null Frame (5) SUBCLASS: N/A (0) ] [SIP/1001-00000004]