www.voip-info.org has lots of info. I suspect that G400 may have enough [em]raw[/em] compute power, but not in C# code. Maybe in RLP. The SIP protocol could probably be done in C#, but the data streams likely would be a problem. Additionally, even if you decode/encode fast enough, you’ve still have to get the decoded data to an audio output device, which given the other constrains would probably have to be done in RLP as well.
Which OLinuXino board/design are you talking about? Quick scan shows many of the boards are 1Ghz.
You said C# + Mono, are you handling the data stream decoding/encoding & Audio all in that environment? If the answer is yes, and you are using the 454Mhz board, logic says it is doable - because, hypothetically, with Mono sitting on top of Linux, you’ve got additional overhead in that environment, that wouldn’t be present on the G400.