[Freeswitch-users] How to set ptime in SDP

I'm trying to send 40 ms of speech in each RTP frame  when working   with a specific/certain  gateway . By means of modifying [var.xml]: <X-PRE-PROCESS cmd="set" data="global_codec_prefs=PCMA@8000h@40i,PCMU@8000h@40i"/> <X-PRE-PROCESS cmd="set" data="outbound_codec_prefs=PCMA@8000h@40i,PCMU@8000h@40i"/> , it is possible to call FROM this gateway (gateway does not send ptime in SDP). But, when the call comes from the other client with ptime set to 20 in SDP, outgoing INVITE messange TO this gateway contains ptime set to 20. How can I set ptime to 40 (it's desirable to do so only for one specific gateway). With best regards, Shumaev D.A., programmer @ KBR Ltd.