Hello Daniel, the problem is permanent. I identified that the problem is reproducible all the time, but only with the mobile numbers from the network of the Bulgarian operator A1 (the operator prefix is 88) - https://www.a1.bg/en.
For instance when dialing this mobile number: +359 88 9655505, there is no any dialing tone and just silence. The situation is the same with all A1 operator mobile numbers I tried to call in the last week.
Note: +359 - is the Bulgaria’s international dial code, 88 - the dial code/prefix served by the Bulgarian mobile operator A1.
At the same time, if I try to call to a mobile number of another mobile operator in Bulgaria like this one: +359 89 676 3187, there is no problem. Note: the Bulgarian operator serving 89 prefix is Yettel.bg
So, it seems that the problem is limited to the connection between UPC Mobile/Sunrise and the Bulgarian mobile operator A1.
Thank you for your help in advance.