Mobile giants Vodafone and Orange recently began offering their subscribers Nokia’s flagship multimedia phone, the N95. It’s a great handset, boasting GPS, a five-megapixel camera, video capture, digital music player, Wi-Fi and Bluetooth. You would think Vodafone and Orange users would be tickled pink, and many probably are. But some are far from happy.

Why so? Well, as well as all the above, the N95 is also designed to support the Session Initiation Protocol (SIP), which underpins a lot of voice over IP (VoIP) services. Vodafone and Orange, however, have decided to prevent their customers from using the N95 to access these services by disabling the phone’s SIP support.

The response from some users has been furious, judging by posts on Nokia’s support forum. It appears many people bought the N95 specifically because they thought it would allow them to set up a SIP-based VoIP service to run across the Vodafone and Orange data networks.

According to reports, the two carriers differ slightly in their motives for preventing this. Orange seems to be saying that there wasn’t time to test the service before rolling out the phone, while Vodafone apparently believes that the technology still has problems and is not mature enough yet. Well, why not let customers be the judges of whether this technology is mature or not.

It is hard to see what the carriers thought they would gain by their denial-of-service antics. They have successfully alienated a bunch of customers, some of whom will no doubt decide to jump ship to rival operators. Others, meanwhile, are hitting back by re-flashing their phones with the original Nokia N95 firmware.

This row is highly illustrative of the growing VoIP dilemma facing carriers: namely, how to satisfy the rapidly increasing demand from customers for VoIP without fatally undermining voice revenues. BT, for example, allows broadband users to make free IP calls using its softphone to other BT softphone users, but charges for IP calls to normal landlines. BT knows that the relative inconvenience of having to download software and use a PC helps to protect its revenues from residential phones.

Mobile operators, however, cannot use the “convenience” defence. Their only source of comfort is that currently the quality of mobile VoIP is so poor. (Let’s face it, it’s still pretty ropey on wired networks.) However, as the wireless operators upgrade their networks, quality will become less of an issue, and then the fight to hang on to voice revenues will really be on.