SIP-415-Unsupported media Type

I was testing two MTAs , one is an eMTA with DOCSIS cable modem and one is "Stollmann ITA" which is a SIP MTA with ISDN phone interface.

I had three SIP endpoints ,

1) x-lite soft phone in my Notebook
2) eMTA
3) Stollmann ITA

-> I could call both eMTA and Stollmann ITA from the soft phone I have in my notebook but the eMTA and Stollmann ITA could not call each other , First I though may be some route is missing but it was not the case , because all three endpoints could register with the SIP server. Then I captured the SIP packets at the SIP server and found the message "415-Unsupported Media Type"... ha ... got it,

Then I checked the capabilities of two devices ,

eMTA - Only supports PCM u-Law









Stollmann ITA only support PCM A-Law











The x-lite soft phone supports many media types , thats why It could call and receive calls form both












you can find these info in the SIP invite message ,

SIP --> Message body --> Media Attribute