Much of the talk around SIP has focused on its role in making IP telephony (IPT) interoperable, but it's rather more than that - and it's not quite that simple - according to Carolyn Nguyen, director of global mobility strategy at telecoms supplier Avaya.

With her company announcing a set of SIP-enabled telephony products and SIP upgrades this week, she is at pains to point out that SIP is not a replacement for IPT. Instead, what it replaces is H323 as a method for signalling, and it's not just for telephony - it enables a variety of applications.

"Telephony is just one means by which people communicate, and to do collaboration you need more than that," she says. "SIP is how you do the signalling but it's more than that. SIP is a simple message requesting contact, then you use it to negotiate the communications, but the protocol used is different, for example IPT or instant messaging.

"The way we use SIP is to say people won't let go of today's functionality. SIP doesn't support enough mainstream features to replace today's IP or PBX phones, so our aim is to use it to add on to IP telephony. For example, if you have a softphone application, you can enhance it with SIP-based presence awareness."

Presence - the ability to see who's at their desk before picking up your phone, just as you expect to be able to see who is online before sending an instant message - is going to be an important benefit of SIP.

Nguyen adds: "It's not just about presence though, it's are you available and who to? How do you define availability and set it?

"We will start to see presence converging with mobility," she continues. "For example, we have built a trial system for nurse-call in hospitals, the aim is to find the closest nurse or doctor to the patient, so we map the hospital and use Bluetooth to locate people."

She adds that where SIP does enable multivendor interoperability for telephony, in the current generation it is only lowest common denominator interoperability. For example, if you use a feature that allows your assistant to pick up your calls, that might not work via SIP.

Nevertheless, as an established standards-based framework for presence, its future is assured: "The mobile operators have adopted SIP for signalling too, for their fourth generation systems as part of 3GPP. Lots of Wi-Fi phones use SIP too in place of H323," Nguyen says.

Turning to the role of mobile phones, and Avaya's plans to integrate them with fixed-line enterprise telephony, she reckons that the main driver for mobile phones will continue to be voice for some years yet, but says that there are still plenty of opportunities to add more.

"Voice is the killer app but enterprise voice budgets are flat and it's data that's growing," she says. "The first step is PBX functionality on the mobile, the next is to enhance it with, for example, conferencing as an enterprise service - we have a feature called meet-me conferencing, it allows up to six people because most conference calls are six or less.

"Few companies will go mobile-only," she adds. "People like traditional phones because of the separate handset, big keyboard and speaker-phone. Going forward, more will use mobile phones but the percentage will differ by geography."

And she says that with voice over Wi-Fi well on the way, along with converged GSM and wireless IP phones, a key factor will be how the mobile network operators react to the prospect of flat-rate competition. Will they look for new co-operative opportunities, or retreat into a control freak mentality focused on voice call minutes?

"There's neat things happening within operators to understand fixed/wireless convergence, and there is starting to be recognition from the operators and handset manufacturers that enterprises have different needs, and that enterprise handsets might be different," Nguyen says.

"It's going to take someone from outside to see the opportunities. We found negotiating with Nokia that there's very different mindsets involved, and it's very difficult to bring them together.

"The enterprise wants to control everything, but the operator controls the network and nothing happens on it without the operator's permission. Each thinks it can do without the other, but to get presence working they have to come together."