Digit drop at dialing on FXS with a fax machine (Ricoh MPC)

I’ve got a strange problem with my Asterisk server (FreePBX Distro).

Whenever I try to dial out via my FXS via our fax-machine (Ricoh Aficio MP C3300) it drops most of the time some digits (most often the “2”) in the called telephonenumber. If I dial two times the “2” in the number, it works.

For example: I try to dial the following number:

0815880254 it dials only 081588054 or sometimes 08588054 (drops the 1 and the 2), as I can see in the CDR-records on Asterisk.

I guess, my fax-machine dials too fast? Is it a problem with DAHDI or my FXS Port? Or of Asterisk?

If I dial each digit by hand with an attached phone on the fax-machine, there is no problem. So I looked for changing the “dialing speed” on the fax-machine, but I couldn’t find such an option. Is there a way to change a setting on Asterisk/DAHDI/FXS port, so it will listen “better”?

Thanks so much in advance for your appreciated help