show version
show dial-peer voice

debug fax relay t30 all-level-1
debug fax relay t30 called-number <number>
debug fax relay t30 calling-number <number>
debug voip ccapi inout
debug vtsp all
debug voip rtp session nse

>на 2621
>
>dial-peer voice 1000 voip
> description fax
> destination-pattern [12345789]....
> voice-class codec 1
> session target ras
> dtmf-relay h245-signal
> fax rate 14400
> fax protocol t38 ls-redundancy 0 hs-redundancy 0 fallback cisco
> ip qos dscp cs5 media
> no vad
>
>dial-peer voice 222 pots
> destination-pattern 222
> direct-inward-dial
> port 1/0:15
> forward-digits all
>
>
>на 5350
>
>boot system flash:c5350-is-mz.122-2.xb15.bin
>
>voice service voip
> fax protocol t38 ls-redundancy 0 hs-redundancy 0
> h323
>
>voice class codec 1
> codec preference 1 g729r8
> codec preference 2 g711alaw
>
>
>interface Serial3/0:15
> no ip address
> isdn switch-type primary-qsig
> isdn incoming-voice modem
> isdn send-alerting
> isdn sending-complete
> no cdp enable
>
>dial-peer voice 1000 voip
> huntstop
> destination-pattern .
> session target ras
> dtmf-relay h245-signal
> fax rate 14400
> no vad
>
>dial-peer voice 111 pots
> destination-pattern 111
> direct-inward-dial
> port 3/0:D
> forward-digits all

попробуй

fax protocol t38 ls-redundancy 5 hs-redundancy 2 fallback cisco
или
fax protocol t38 ls-redundancy 3 hs-redundancy 0 fallback cisco

в обе стороны!