We use them a fair bit, 95% succesful.
You MUST use G711a/u or T38 (better), any thing else and you are really taking a risk. Oh and good QoS
You'll need a decent ATA, avoid the cheaper ones, we find they fall over to often.
Then it's case tweaking the settings until it works.
Personally I hate the ******** things. As stated above modems are even worse, we get about a 20% success rate.
Most people spend their time on the "urgent" rather than on the "important."