I will try and help you backwards! You need to first set up forwarding on voipcheap to your UM telephone number. Do you know how to do that? In the client, go to the 'Tools' menu and select 'Options'. There is a setting there, where you can enter a forwarding number. Remember to select "Always" for when you want the call to be forwarded. Any incoming SIP call that comes to your voipcheap account if you are online or not will then be diverted to your UM SIM. You will charged the standard rate. Incoming calls can come in via your SIP address which has a format like
voipcheapusername@voipcheap.com.
I don't use Eutelia or pbexes but I think you can bypass pbxes.org and get Eutelia to point incoming calls from your DID directly to any SIP adress. In this case, just enter the one in the format above into your Eutelia options. (or you may need to enter SIP:voipcheapusername@voipcheap.com)