Thanks for your input it helped me a lot.
I've replaced all our USR modems with Multitech 5600ZDX and a lot of
errors that resulted with :AT+FTH=3 returning nothing (ie.)
<-- [9:AT+FTH=3\r]
--> [0:]
Have decreased a lot. But I still get this error with the multitechs
but not as often as with the USR modems. I might see this kind of
error at most once a day. There are days where I don't see it at all.
Is this something related to improper or odd signaling from the sender
that just can't be helped on my side ? or would it be something else ?