Subscribers to du's fixed and mobile internet connections yesterday found they were able to access Skype calls to mobile phones and landlines.
Although the block to the Skype website previously made it difficult to install the service on computers in the UAE, it was available for smartphones from foreign-based applications shops, and once installed it was possible to make voice and video calls.
Etisalat users are still blocked from accessing the website or making Skype-to-phone calls.
Those who try to access the site find the message: "Access to this site is currently blocked. The site falls under the Prohibited Content Categories of the UAE's Internet Access Management Policy."
But du subscribers sail straight through to Skype's homepage where they can sign up for an account, add credit and download the application.
It was unclear when the block was lifted, although discussions on several internet forums indicate this is not the first time it has happened.
It is also unclear whether it was permanent, or whether it was an intentional move by du.
Worldwide, Skype has more than 31 million registered users and the attraction of free international Skype-to-Skype calling is clear for the UAE's expatriate majority.
Although it is classified by the Telecommunications Regulatory Authority as an "unlicensed VOIP" (voice over internet protocol) service, the TRA has since 2011 stated there is nothing stopping du and Etisalat from licensing Skype or similar services.
The TRA website states the use of such software may be illegal unless permission is sought from the licensee, in this case Etisalat or du.
But it adds: "There are no regulatory or legal barriers to the licensees providing VoIP services, nor is there any specific requirement for the licensees to offer VoIP services within a certain time frame.
"We suggest that specific questions about the timing or type of VoIP service offerings be addressed to the licensees."
Etisalat, however, said on its Twitter feed, @EtisalatUAE, that Skype was "categorised as unlicensed VoIP, thus banned as per local regulations".
Skype has said in the past that it does not require a licence, as it is free to use.
Etisalat and du would stand to lose significant revenue from international calls if they allowed the system to be used on their networks.
And Etisalat and the Emirates Integrated Telecommunications Company, which owns du, offer prepaid VoIP phone cards, with lower rates than their standard phone charges.
Etisalat's card, Five, was launched last month while its rival, Hello, has been available since 2010.
A spokeswoman for Microsoft, which bought Skype for US$8.6 billion (Dh31.58bn) in 2011, said the company was "not aware of any changes in regulation and we have not been informed of any. We usually follow the cue of the local TRA or operator."
No comment was immediately available from du or the TRA.