It depends upon a lot of factors.
You could have your voicemail messages emailed to you and listen to them on your telephone without having to pay your SIP provider.
You could also install a SIP client (like CSipSimple) and have it register directly to your PBX via a VPN. But, that's fairly complicated to set-up.
If your SIP provider allows free calls from its own customers to its own customers, you could set-up another account on your SIP provider and have your SIP client on your phone (i.e., CSipSimple) register to that, and then use that to call your DID for free. Callcentric allows for this, I believe.
