Dtmf for outgoing calls

I’ve asterisk 1.8.20.0-0. Configured that PBX calls a mobile phone and then mobile should dial number (standard callback + disa). and that moment some digits are lost randomly. The number delivers fully to the PBX, and then digits are lost. What should I check?

Asterisk 1.8 is well past the end of its support life.

DTMF handling on mobiles is complex. Can you confirm that the number is being dialled from the phones keypad? Any attempt to inject DTMF directly into the audio would almost certainly fail in this sort of way, as GSM is not suitable for tone signalling; the tones need to be sent specially.

This is typical for in-band dtmf. It might lost digits some time, especially if the codec is not G.711.
Configure DTMF method to be RFC 2833 or SIP INFO. (Not in-band!)

solved!

added
relaxdtmf=yes
rfc2833compensate=yes
to the sip.conf