Incoming number sometimes stripped

Hi folks
I really need someone more experienced to point me in the right direction with this one. We’re using asterisk 1.8.23.1 and everythings fine except the fact, that some incoming pone calls (through dahdi, Sangoma AFT-B700) are missing their last digit. Fe. assume our main number is 123456 with further extensions from 10-100. Every now & then some known partners dial fe. 16 and I see it in the log incoming as 1234561, then given to the invalid handler (ext 1 doesn’t exist) and passed to our secretary. Of course I will paste the responsible dialplan if needed, but since this does not happen as a rule, I doubt logical dialplan mistakes.
This is an example log entry:

[Apr 24 12:43:23] VERBOSE[20853] pbx.c:     -- Executing [1234568@vom-amt:1] Goto("DAHDI/i4/xxxxxxxx-377", "8,1") in new stack
[Apr 24 12:43:23] VERBOSE[20853] pbx.c:     -- Goto (vom-amt,8,1)
[Apr 24 12:43:23] VERBOSE[20853] pbx.c:     -- Sent into invalid extension '8' in context 'vom-amt' on DAHDI/i4/xxxxxxxx-377
[Apr 24 12:43:23] VERBOSE[20853] pbx.c:     -- Executing [i@vom-amt:1] Goto("DAHDI/i4/xxxxxxxx-377", "intern,0,1") in new stack
[Apr 24 12:43:23] VERBOSE[20853] pbx.c:     -- Goto (intern,0,1)

I am not seeing the whole number dialed (should have been 87). Could there be anything else causing this behaviour than dialplan logic ?
And yes of course, in the beginning I blamed the caller …
thanks in advance.
Ronald

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