As mentioned, you can make international calls from a regular landline phone thru your OBi by setting up an incoming number on your OBi adapter to access the built-in Automated Attendant (aa) which has an option to bridge the dialed call using a voip service setup on your OBi. This makes it more complicated to use that incoming number for other uses. You can, though, setup Speed Dials on the Obi to minimize the dialing and make it more foolproof.
You can also make international calls from a regular landline (or cellular) phone by setting up 2-stage dialing with some provider. That is you call one number, get a dialtone and dial the second number. Many providers that do this have a "phone book" setup where the 2d number you dial can be some shortcut number that you have setup to dial the complete number. You can usually set these up so that if you are calling from a known number (your home phone for instance) you do not have to enter some authenticating password or calling card number.
You could get your mother her own
GoogleVoice account (number) and if configured correctly (and funded for the international calls) she could then call that number from her home phone, enter 2 at the prompt, and then dial the international number.
Other choices include getting a local number from another provider and setting it up for "calling card" or "DISA" service, or calling a local access number that is shared by many.
Skype will give you local preset numbers for up to 30 of your international friends. You call the local number and it rings the international number you have setup.
OneSuite has general local numbers or a toll free number that you can call for their "Calling Card" service. With
CallCentric you can setup a purchased DID for "Calling Card". With
Voip.ms you can setup a purchased DID for "DISA".
Anveo can be setup for "DISA".
Future-Nine has a number of local access numbers to access their "Calling Card" service. You can also access Future-Nine's calling card service with some additional dialing thru SipBroker.