rtp audio only over web.de?

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Hello,

the following situation:
I telephone to 488 between linksys spa-3102 and grandstream, with web.de. I cannot use unfortunately the low bit rate codec, since it apparent only supports web.de iLBC of it, and spa iLBC cannot.
But spa g.723 and g.729 can, grandstream know also these codecs. Apparent rtp audio is only unfortunately led over web.de, and not directly between the adapters.
And web.de can only:
Quotation:

m=audio 10866 RTP/AVP 8 0 2 3 97 101 a=rtpmap: 8 PCMA/8000 a=rtpmap: 0 PCMU/8000 a=rtpmap: 2 G726-32/8000 a=rtpmap: 3 GSM/8000 a=rtpmap: 97 iLBC/8000 a=rtpmap: 101 telephone-event/8000

As I create it, the audio directly between the adapters
one transfers, so that I use the low bit rate codec
can?