Usually a cellphone will ring only once and then go to voicemail when someone has blocked you.
Or the cellphone may not ring at all and may immediately go directly to voicemail without ringing.
It depends on the cellphone company and how the person has their cellphone blocking setup.
I've had some cellphones that were blocked tell me I was blocked while others just rang once and went to voicemail and others rang 2 times and went to voicemail.
Sometimes though a cellphone may ring once and go to voicemail when the cellphone is shut off or the person has it in airplane mode.
Or the cellphone may go directly to voicemail when the cellphone is off or in airplane mode.