Your description of the failure is a little vague so with out code to look at we can do little else but guess. Even with the code I am not sure we can help since it is not clear what "fails to do so" means. No disrespect intended, but in your mind "fails to do so" is clear to you, but we, on the outside, are lacking context to understand.
So questions:
Does "fail to do so" mean:
1) the modem init string does not appear:
2) the HAL call returns !success,
3) the interrupt that would respond does not happen?
Last of all when you say "initialization was successful", again we are lacking context. The modem, the moden init string, or something else? The HAL call you mentioned is supposed to occur after UART init, and the HAL initialization code for the UART IT handling, and does not really do any init functions other than prime the peripheral to look for idle and receive data up to that point, or receive data up to the count limit. Also need to know, are you using a circular buffer or normal.
I hope this does come across as being critical; I am just trying to understand the context.
