About 700 people have now complained to watchdog Ofcom they were unable to vote for Rhydian Roberts in The X Factor.
The 24-year-old Welsh favourite was beaten to the title in Saturday's ITV live show by Scotland's Leon Jackson.
Ofcom said complaints had risen dramatically. Welsh First Minister Rhodri Morgan has revealed he was among those who tried and failed to back him.
The programme's production company said the high number of calls meant some people were greeted by engaged tones.
An Ofcom spokeswoman said: "We have now had around 700 complaints from people who said they were not able to get through to vote for Rhydian.
"We are now working with [phone regulator] PhonepayPlus to investigate the matter."
Ofcom said there was no timescale for the completion of its investigation.
Before Saturday's final, Rhydian was clear favourite, but he was pipped at the post and a £1m recording contract by Scottish crooner Leon, who sang a duet with Kylie Minogue on the live show.
A stream of viewers contacted the BBC to complain that they could not get through and some radio stations in Wales said they would boycott Leon's single and demand a recount.
Rhodri Morgan told BBC Radio Wales he had been unable to get through to the programme.
"I can confirm that at around 29 minutes past 10 I attempted to vote for Rhydian and I couldn't get through, so I think if thousands of people had the same experience as me..." he said.
Mr Morgan admitted he did not hear Rhydian, of Sennybridge, Powys, perform.
"But I just thought as a loyal Welsh person I should put in my vote for him, but it couldn't record my vote so my telephone bill will be smaller," he said.
"But if there were thousands of people like me, I don't know what happened...
Viewer Giles Davies wrote to the BBC News website: "I'm just contacting you to let you know that my family and I were another 'victim' of the blocked ......... sorry, busy phone lines on Saturday.
"We didn't get through once during the final despite trying probably 15-20 times."
Another Rhydian supporter, Angharad Howells, said she was unable to register her until the last few minutes of the show.
No reported faults
"I think there should be a recount and this matter addressed. I will certainly not buy the winner's new single in protest against a very peculiar event."
X Factor production company Talkback Thames admitted some viewers trying to register their votes were met by the engaged tone, but blamed it on the sheer volume of calls.
"Some Virgin Media customers did experience problems getting through because of high call volumes, but there were no other reported faults on the lines."
A Virgin Media spokesperson said: "We are currently looking into the X Factor matter but so far have not found anything unusual.
Source
X Factor voting complaints soar
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