Hi... we have the same issue :-(
Are you using 3CX?
After how much time does the issue occur after reboot?
We have an existing issue (SCPP-8179) and we're working on it.
In the meanwhile can you try to change the following setting: http://wiki.snom.com/Settings/user_dtmf_info to value "sip_info_only" or "on" ?
We have the same issue with D375 and 3cx. 188.8.131.52. But i do not find the DTMF setting.
The setting is here: Identity X -> SIP -> DTMF via SIP INFO