A Working Lunch viewer got in touch with us after his sister was charged £4 for a short telephone call to an 070 number.
070 numbers will be phased out from the end of this year
What looks like a mobile number is in fact what Ofcom calls a 'follow me' or 'personal number'.
It can be set up to divert calls from one number to another, so that the person being called can keep their own number private, and remain contactable wherever they go.
Numbers starting with 070 can cost up to £1.50 per minute and are normally used in classified adverts, patient lines in hospitals and by internet chatroom users.
Joe Delaney contacted the programme because he was worried about the cost of a call his sister made to an advert.
"She was barely on the phone for just over a minute yet the call cost her £4, so extrapolate that over the hundreds of people who buy Loot every day looking for a bargain. Someone is making money somewhere and it just doesn't seem right."
Loot says it will look into what has happened in the case of Joe's sister.
The classified adverts paper allocates a 'Safe Call' number to sellers who use their Homes and Gardens section.
"'Safe Call' allows our advertisers the security of keeping their own private telephone number out of the public view," Loot told Working Lunch.
Numbers starting 070 also allow callers to leave a message through a voicemail service, and they enable the advertiser to 'turn off' the response once the item has been sold.
A charge of "32 pence per minute from landline or mobile phone" is explicitly stated alongside adverts using a 'Safe Call' number.
However, Loot says that they cannot comment on whether it is 32 pence or not.
Ofcom says that 070 is not a premium number like those starting with 09, which permits revenue sharing between telephone companies and call recipients.
Loot says that 'Safe Call' numbers give their sellers security
"If Ofcom receives complaints alleging misuse of numbers including 070 numbers we will look into these with a view to a formal investigation based on the evidence submitted," the regulator said.
The numbers are due to be phased out from the end of the year, with an 060 number replacing it if there is enough demand.
The price of the new number would be set at around 20 pence per minute, with the caller being warned at the beginning of each call exceeding this tariff.