Hi Tribu, You don’t mention what particular Quintum you are using but unless it is a CMS or the new DX Series it won’t do CODEC auto-negotiation.
This means that you will have to set the CODEC to be exactly the same as the CODEC you are using on the Cisco. You have to be specific here, the Quintum does not work well with the Cisco CODEC G729R8, use the BR8 algorithm instead (G.729BR8).
If you are using the CMS or new DX range you can set up auto-negotiation and CODEC preferences.
I hope this helps, if you need any further assistance please feel free to contact me at mhitchcock@blue-lava.co.uk