Maxcom MM41D User Manual page 24

Please make sure the network supports the call waiting
function and the status has been set to Enabled.
If there is a second incoming call in the process of the
first call, you'll hear the "Beep" tone and the screen will
prompt.
Press [Left soft key] to answer the second incoming call.
When the second call is put through, the first call will be
switched to held state.
Press [Power key] to terminate the second call.
Select current phone number
When the second call is answered, press [Send key] to
swap between the current call and the held call, and
press [Left soft key] to [Options] menu:
Swap: to hold current call and activate the held call
Merge: Add held call to conference call.
End call: Terminate current call.
End Merge: Terminate conference call.
Note: Not all networks provide the multi-party
service.