ASTERISK without audio

when I bind for a remote telephone does not have audio but it has Ring and I receive the following message, I use the VONO operator:

[Apr 9 11:46:14] WARNING[6001]: chan_sip.c:3669 sip_write: Asked to transmit frame type 4, while native formats is 0x100 (g729)(256) read/write = 0x100 (g729)(256)/0x100 (g729)(256)

they could help me?