That's a good & fun question; generally, there's a worldbuilding StackExchange sister site especially meant for people trying to build coherent universes. Check it out!
Anyway, your question can be reasonably interpreted as "if we have a sporadic transmitter that ground can't schedule, how would it communicate with ESTRACK"; and that's a serious SX question, so here we go:
There's ESA PSS-04-105, Issue 2.4 (November 1996) which defines the carrier frequencies, rates, bandwidths and modulation types of transmissions within ESTRACK.
For something like textual messages, that network wasn't actually meant – it's designed to primarily transport telemetry, and do some ranging, and nobody's expecting/decoding/guessing there'd be any English text among these numbers. But: guessing that if you send numbers that clearly aren't from normal operation, you'd be able to actually get an engineer to look for sense in the data received. Then they'd start to guess what it means. Chances are you're best off transmitting it as bits as used in the common (and old) ASCII encoding (which maps letters to binary numbers). At some point, people start recognizing that the numbers representing
r are pretty common...
But: to first get the receiver to actually see data, you'd have to build a transmission that is "sound". Basically: Radio transmissions are enriched with what us communication engineers/ information theorists call "redundancy". Think of that as forms of checksums. For example, a common checksum would simply be the sum of all bytes sent (a byte can take values between 0 and 255), modulo 64. You just send your message and attach that checksum. The receiver checks whether it gets the same checksum as you sent, and if it did not, it knows something about the transmission is broken (and would ignore it). If you are cleverer than simple checksums, you can build codes that allow the receiver to "repair" a limited number of errors.
Space communication engineers are clever. The problem with that is that the codes can get very long and it can be very hard to guess how to build a valid transmission without their knowledge. So, that would probably pose a reasonable challenge for your aliens.
However, since that's all grounded in math, and math is the same all over the universe, and your aliens are advanced and definitely will have some form of codes since they need to communicate using radio waves themselves, that'd basically be a challenge in understanding how this crazy human mind understands number and ring theory, or, more generally, algebra. It will require that they either bring their mathematicians along, or that they have computers onboard that were programmed to solve such hard problems (us earthlings are desperate to find better and more codes than we know – basically, we have a rich, well-proven theory of what great codes must exist, but we simply know none of them, although there are very, very many), which makes it likely that they have some kind of quantum computer (just don't quote me on quantum computing – it's just that the computers we know aren't any good at solving that kind of problem, as our lack of code knowledge demonstrates). Which in turn means that it should be relatively easy for them, given enough recording of e.g. TV signals, figure out how to synthesize TV signals containing human speech/imagery. (not the picture, they've never saw what a TV screen shows, just the signal) Think of these funky deep neural networks transferring e.g. Van Gogh's style to your selfies. You can "deep dream" valid TV signals, too.
Thus, maybe they'd much rather directly interact with TV receivers on earth, sending disturbing images first, maybe things that look like strangely combined images that dominate TV transmissions, but get better the more they can actually observe what TV signals the humans reply with, once they figure out there's an extraterrestrial source of those.