Expect some very poor timing decisions in various designs.

The preamble for V.21 HDLC data is specified as being 1s ±15%. This should mean 850ms of preamble is within spec, and many modern FAX machines only send that much. A number of T.38 implementations choke on this. They require at least 1s between the v21-preamble message and the first HDLC data, for reliable operation. This is very poor design. Even if there is 1s between these events at the source, jitter on the network can make them arrive less than 1s apart. It appears the best a tolerant implementation can really do is impose a 1s minimum between sending these T.38 messages, and accept that things might still go wrong when there is some jitter.

There is similar intolerance with the timing of the start of training messages for the fast image data modems. Most implementations do not impose a minimum which is above that permitted by the T.30 specification. Some do, however, make no allowance for network jitter reducing the interval at the receiver. You should expect the far end to be using TEP, when sizing the required delay between the start of training, and the first data. Also, for ECM data, make sure the delay allows for the specified minimum 200ms of preamble.