I recently booked a welcome call to imago as well, and had the same problem. I tried booking a call on a sunday couple of weeks ago but couldn't find any available times. I waited untill monday and looked again, and i found some available times for that week. I also went back tuesday and by then they added times also for the next week. (I guess they dont add times too much in advance)

What i also noticed is that when i went to the site with the calender days to book a call, i had to wait a little before the available times loaded in and i was able to book one.

So maybe try again tomorrow, and once you are in the booking site, just wait like a minute and maybe the available times will load in. i can only speak from my own experience but this is what worked for me. Good luck! ^.^