Hi!
You can get devices that (I believe) can only be used to call for less than that from Fleabay 
or Chinese websites…
I don’t know how good these are, though…
But using a Raspberry Pi decreases it…
Don’t get me wrong, I love Raspberry Pi’s and I have a few but a SD card is not the most reliable storage media…
Now, installing the dongle directly on the PBX would be a lot more interesting…
Good luck and have a nice day!
Nick