How to configure or modify FreeSWITCH (In BBB) for participants with low bandwidth?

[solca]

If attendees do not have a internet connection high bandwidth,  on which aspects should be taken into account to obtain an acceptable quality in the management of voice?

What codec should be used preferentially FreeSWITCH?

Originally vars.xml have this:

 <X-PRE-PROCESS cmd="set" data="global_codec_prefs=speex@16000h@20i,speex@8000h@20i,G7221@32000h,G7221@16000h,G722,PCMU,PCMA,GSM" />

<X-PRE-PROCESS cmd="set" data="outbound_codec_prefs=speex@16000h@20i,PCMU,PCMA,GSM" />

Questions:

1. What should be modified to use a codec that works well with low-band width?  (and that the voice quality is acceptable)

2. What others tips could we give?

Thanks in advance for your replies.

Solón Cáceres