In the MIVO250 programming, the call configuration used for the 112 Dect devices, make sure that the DTMF Encoding settings is set for - RFC2833
Under the configuration of the SIP phones make sure DTMF Payload is set for 101
In the Dect 112 programming under Servers make sure the following are set
DTMF Signalling = RFC 2833
DTMF Payload Type = 101