HMR for adding ptime attribute into SDP

Hi all, 

I'm actually looking for a solution to insert "a=ptime:20" into the SDP body, but only if SDP contains:

- no ptime attribute at all

- PCMA or PCMU codec

 

Now I have added this rule:

match-value: (a=rtpmap:[8|0].*)
new-value: $1+$CRLF+"a=ptime:20"+$CRLF

 

and it works so far. But it inserts two ptime attribute lines (one for PCMA and one for PCMU) if SDP contains both codecs.

Is it possible to create an element rule which fits my requirements?

 

Thanks and Regards

Heino