I looked at PCAPs for concerned calls, and saw that DTMF tones are only sent with RFC2833, not with SIP INFO nor InBand. 
I first thought that digits may be received simultaneously with RFC2833 and InBand but definitely not.

Is that end-to-end, or are they reaching you via someone else? Or something else (ATA adapter)? If so perhaps some inband->RFC2833 is happening between you and them that's introducing the bad digit.

If not can you share the PCAP (or just its RFC2833 packets)? That might explain what's happening.

Do you think what could be the cause of that ? I don't use  "start_dtmf" in my diaplans, I already saw in other ML topics that this may cause double digit problems.

That's only if there's inband and out-of-band (RFC2833/INFO) DTMF at the same time. There's no issue otherwise, but using it on a call without inband DTMF is a waste of CPU.

If you're getting RFC2833 then any inband audio will only cause double digits if you run start_dtmf, inband will be ignored by default.

Is there a way to ask freeswitch to only handle 0-9*# characters ? Or any idea, I'll be glad to try anything that can help us !

The problem is RFC2833 will be saying the 1 digit finished before the C one started, FS can only interpret it as its told to.

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