Packetization Rate with IAX Softphones

Hi, I would like to know if there is any iax softphone which I can change the packetization rate for diferents codecs. For example, right now I’m using Linphone for SIP account and it gives me the posibility to modify the SDP parameters to change the packet rate (ex: G.711 20ms to G.711 30ms).
I’m using the field “allow” in the PBX but my current softphone doesnt use this instruction, so I need to change it into the softphone too.

Hope you can understand.

Thank in advance.