No problem here with the Obi dropping calls from land-lines, my wife would have a lot to say if that happened.
As for the price of calls, around £10 for 4 months of free calls to 01, 02, 03 numbers and mobile calls at 1.2p per minute.
Note: Just remembered, I did have a problem with VoIP when using Plusnet. Needed to get a static IP address from them to fix the problem.