Fax and Codec Error

I have the following problem when I send fax to asterisk:

 -- Executing RxFAX("IAX2/1002@",

“/var/spool/asterisk/fax/_1125039307.1.tif”) in new stack
2005-08-26 06:55:09 NOTICE[30852]: channel.c:1317 ast_read: Dropping
incompatible voice frame on IAX2/1002@ of format
slin since our native format has changed to gsm

Reading around, I understood that I would have to add `allow=slin’ to
iax.conf, but this hasn’t changed anything. The dis/allow part of my
iax.conf is:





But this didn’t change anything.
Then, I was told that I should comment the `allow=gms’ line. But this
made things worse. Now, when I dial asterisk I get the following error:

2005-08-26 07:50:44 NOTICE[31232]: chan_iax2.c:5783 socket_read:
Rejected connect attempt from, requested/capability
0x8/0xff0f incompatible with our capability 0xf840.

I don’t know much of how codecs work or how to configure asterisk, so
please help