Asterisk newbie - sip config

Hi all,

I’m getting this error when making a sip call from the zap channel.

sip_write: Asked to transmit frame type 4, while native formats is 256 (read/write = 256/256)
May 25 15:09:59 WARNING[26003]: chan_zap.c:4715 zt_write: Cannot handle frames in 256 format

what does it mean and how can I correct it?

Thanks in advance,