Ringgroup of classic extensions stops immediately when one busy or goes to voicemail

I have a ringgroup of "classic" extensions configured with ringall. For a year it worked well, but now I have the following problem:

As soon as one phone is busy and/or goes to voicemail, it gets connected to the voicemail and the other phones stop ringing.

In the past it would ignore the voicemail and just keep ringing the other phones.

I have a "SOHO" account.

Can anybody help me? Because now PBXes has become unusable for me.

Thanks,
Steve