IMO, if this is going to work at all, you've got to get the receptionist out of the picture altogether.

For all you know, she's not relaying the message, he doesn't check in with her OR she might even decide that she gets to play triage and decide which calls are important and which aren't.

This really is something that your therapist and you need to work out. It's either okay to call and expect a call back, or it isn't.

This limbo has got to be terrible, and I'm not sure you have to put up with it.

Rather than talking to the receptionist directly about it though, I would definitely let the therapist handle it and insist that he leave you out of it altogether. One way to ensure that your calls never see the light of day would be to make her mad.