You will have a Voipcheap 'SIP address' which should be something like this ...
(yourusername)@ voipcheap.com it will be the basis of one of the settings you put into your PAP2
In theory its possible to call another SIP address but you will have problems entering alphanumeric characters from a standard phone. Therefore you could experiment with using a softphone like XLite (free download) on a PC connected to the network and behind the same PAP2. Dependent on the router you use, you may need to open specific ports to ensure you get a two way voice connection.
There is some specific discussion of this topic on the Voipbuster forum here ...
https://forum.voipbuster.com/
You will need to register first to be able to get in.
The other option you have us to use the services available here at Voipuser - if both parties register then calls between Voipuser members are free and unlike Voipcheap you get a free UK Inbound number too.
Please note that despite sharing the same first four letters there is no connection between Voipuser (a VOIP Community) and Voipcheap or Voipbuster (both of which are operated by Betamax Germany)