Media watchdog Ofcom is looking into complaints from X Factor viewers who claim they could not phone in to vote for eventual runner-up Rhydian Roberts.
Rhydian was favourite to win the show, according to Ladbrokes
The Welsh singer was hot favourite to win, but narrowly lost to Leon Jackson on Saturday night's ITV1 show.
Ofcom has had 80 viewers' complaints, and some fans told the BBC they tried and failed to vote up to 10 times.
The programme's production company said the high number of calls meant some people were greeted by engaged tones.
An Ofcom spokesman said viewers had "expressed concerns about the operation of the voting and many said they were struggling to get through".
Some radio stations in Rhydian's native Wales said they would boycott the winner's single and demanded a recount.
Scotland's Leon won the ITV1 show and a £1m recording contract, beating Rhydian, who had been the 1/3 favourite.
But viewers who had tried to vote for Rhydian, 24, from Sennybridge, Powys, insisted the phone lines were constantly busy during Saturday night's live final.
Gwyneth Greenslade, from Llandrindod Wells, Powys, told the BBC News website she had phoned 10 times and could not get through.
Her two friends also tried seven and 10 times respectively - and they too failed to register a vote.
"If you ring 10 times you expect to get through - I was very cross," said Mrs Greenslade.
"A lot of other people in this area said they couldn't get through. It seems to be very unfair."
Andrew Powell, from Abergavenny, e-mailed BBC Wales to say he and his family tried ringing to vote for Rhydian for an hour-and-a-half, only to be told the network was busy.
He wrote: "If the 'network' was busy then surely the amount of votes could not accurately be counted as the possibility of thousands of other voters that could not get through may have altered the outcome."
Fans on website message boards also complained about being unable to get through to vote for Rhydian.
A group of radio stations in Wales - including Bridgend's Bridge FM, Swansea Bay Radio, Radio Carmarthenshire and Radio Pembrokeshire - said they would ban Leon's Christmas single over the voting.
The stations, which are part of the Town and Country Broadcasting group, said they wanted a recount after being inundated with calls from their listeners about the busy phone lines.
X Factor production company Talkback Thames admitted some viewers trying to register their votes were met with the engaged tone, but blamed it on the sheer volume of calls.
"As all numbers go through to the same lines and vote platform, it is impossible for there to be any bias in favour or against a particular contestant," said a spokeswoman.
"Phone lines were monitored throughout the night and there were no issues on voting lines.
"We do get a huge surge of calls and depending on phone operators some callers may have experienced an engaged tone or network busy message and therefore may have to call back, but these calls should not be charged.
"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.
"However, we do know that the large volume of calls meant that some viewers
had to keep redialling in order to get through. This is not unusual, for any of
the telephone networks, for this type of mass voting event."