Imagine only being able to speak to friends on the same phone network as you - nightmare! So why is instant messaging on the net hampered by just such a silly rule?
Send an e-mail to a friend and there's no telling when you might get a reply. They may not check their messages regularly, they may be out for the day, and they may not even use that e-mail address any more.
Texting has thrived thanks to cross-network co-operation
It's the desire to know whether the person you want to reach is there that explains the extraordinary rise in popularity of internet-based instant messaging (IM).
The idea of IM is simple - a program on your computer is tipped off whenever a friend is online at the same time as you.
To send a message, simply click on their name in the IM program and type. Instantly, a small window pops up on their computer screen with your message, and they can type and instant reply.
Huge in America
To do this you need an IM program such as Aim or ICQ, or Microsoft's MSN Messenger or Windows Messenger.
IM is already hugely popular in the US, where internet users spend on average five times more time online than in Europe.
But it is beginning to take off in the UK, with over 3,000 people signing up for MSN Messenger alone every day, according to Microsoft.
Worldwide, AOL's Aim is by far the most popular system: its 195 million users send about 1.6 billion instant messages every day, says the company.
ICQ, also owned by AOL, has about 140 million users, while Microsoft's MSN and Windows Messengers total 75 million users, according to the company.
War of the IM networks
The benefits of IM are clear, but there is one very major drawback - you can only contact friends who use the same IM program as you.
Imagine if the same were true of the phone network - a BT subscriber would only be able to speak to other people with a BT line.
It's enough to drive you crazy
Instant messaging systems are "proprietary" and non-compatible, so if you use Microsoft's MSN Messenger you can't reach friends who use Aim, ICQ, or Yahoo! Messenger.
This is no accident: in 1999 the "instant messaging wars" broke out when Microsoft's MSN Messenger was able to connect to AOL's Aim users. AOL quickly locked out MSN users - to protect its members, it said.
"It was about our members' privacy, and how we open up our networks to others," explains Jane Marrinan, AOL UK's communications manager.
But the real reason each system runs separately is that each owner has a vested interested in doing so, according to Mark Levitt, a vice president at US-based analyst IDC.
Free you network, lose customers
"AOL was the first company to offer an IM product, and has succeeded in keeping its leadership by keeping its system closed. It would be very hard for it to compete with a ubiquitous free instant messaging client like Windows Messenger [which is supplied free by Microsoft with all new versions of Windows].
"Instant messaging, like e-mail, is a great way for AOL (and the other instant messaging companies) to promote affinity for its web portal. AOL would lose its dominant position if it were interoperable."
Live conversation in text, courtesy of AOL
So where does this leave someone who simply wants to reach all their friends? There's no doubt the current system makes IM less useful than it otherwise would be - imagine the hassle if there were numerous incompatible e-mail systems.
IM programs which can connect to multiple networks, like Cerulean Studios' Trillian, present a solution but can be unwieldy because you still need separate accounts with each messaging network.
Fortunately, there are signs that the various IM systems are slowly converging.
AOL has brought its two programs together so the latest version of ICQ can communicate with Aim. And as part of Microsoft's recent anti-trust settlement, AOL said it would "explore ways" to interoperate with Microsoft's IM system.
"It's not 100% certain, but I would be pretty confident that in three to five years there will be a single public IM system like the phone and e-mail systems," says Mr Levitt.
For the moment, it might be easier to phone a friend
If that finally happens, IM operators will have to look at providing extra features to entice users to their particular program.
Some IM programs already offer voice conversations, video conferencing and the ability to swap pictures, music and other files.
Services such as web conferencing, provided by specialist third party software vendors, are likely to become embedded in IM software, while premium paid-for services are on the horizon - AOL is reportedly looking at an IM dating service.
Until then, IM users have one of three options: ensure all your friends use just one program;
run several different IM programs at once; or
try one of the free clients that speak to all networks.
Some of your comments so far:
Trillian is the easiest IM program around. It features none of the bloat of ICQ or MSN and allows you to chat with anyone on any network.
Terry Eden, UK
Convergence is good, but I'm wary of these companies adding "bumpf" that will only over complicate matters for professionals and confuse the home user. Keep it simple or give people the option to keep it simple whilst you bolt on any number of "useful" things.
I don't go online to chat, but it is useful if someone wants to talk to you when the phone line is engaged.
Lizzy Thomas, England
There was a time not so long ago when SMS messages didn't work across networks, only when they were allowed across operators did they really take off into the huge industry they are now.
Suprise suprise, non interoperable systems in the USA. This is almost the exact same problem that they have with mobiles, 4 different systems. This is why they are at least two years behind the rest of the world in the mobile sector
Frank L, UK
Every so often on AIM I get the IM equivalent of spam. If convergence is coming, then we should be given the option of choosing which networks we make ourselves available to.
Colin Scowen, UK
Why can't you just talk to each other! I pick up the phone or visit my friends. Is this messenger service just for the sad and socially inept?
Send us your comments on this story:
Disclaimer: The BBC may edit your comments and cannot guarantee that all e-mails will be published.