Yesterday was the 15th anniversary of the Edinburgh bisexual helpline (RIP). In the past few days, I've had to learn more than I want to about telecoms provision in order to make sure that the London one survives.

After 13 or 14 years of having the London line use a phoneline in his flat, Ian Saxton moved out last weekend. The new people move in next weekend. While I am of course incredibly grateful to Ian for his help over past years — especially when the line operated by connecting two phonelines together with an expensive but appallingly unreliable box of tricks that he had to keep kicking — I would also really rather have liked to have had more than two days notice of the move. Oh well.

For almost seven years, we've been operating with a single line. Most times of the week, callers to the helpline get through to an answerphone in the flat. The message tells them to go away… and call back when we're open. At those times, we use a BT service they now call 'Smart Divert' — we were one of the first to use it in the UK! — which enables us to divert calls to (almost) anywhere else in the world, from anywhere in the world.

So volunteers can do shifts anywhere that's convenient and callers don't know the difference. We pay the cost of diverting the call 'from' the flat to wherever the shift is being done plus a quarterly fee, but that's less than the cost of having two lines. And as the line is only open at evenings and weekends, the diversion call costs are minimal.

Now, with Ian gone, we were in trouble. Especially as I had to remove the answerphone last Friday, so callers 'outside hours' for the past week have only got a ringing tone.

Technically, there is no reason whatsoever why we could not have the line into the flat disconnected (we don't want the new owners being able to answer the phone, much less call out on 'our' line), add something like the otherwise evil Call Minder – a virtual answerphone – and keep going with Smart Divert. BT would make more money – not only do they get the Call Minder rental money, they'd save on having to provide a physical line.

But they wouldn't do that. We 'have' to have a physical line.

We couldn't keep the helpline's number — which has appeared in innumerable places in the the past fourteen years — moving it outside the area of it's current 020 8569 exchange costs a fortune (about £500 per year on top of our current phonebill of about £250). And, incredibly, you can't find out where that area is except on an address-by-address basis. ('12 London Road?' 'Nope.' 'How about 34 High Street?' 'Nope.' etc etc etc) If you do find somewhere, it's £120 for a new line.

The alternative was to have a new number, plus a message on the old number saying 'we've moved, try this number instead'. BT want £25 per quarter for that message, and will only do it for up to a year. That'll mean losing everyone — and there are people who do this — who makes a note of the number then calls it years later.

Then we get onto the choice of which new number to have. An ordinary boring 020 number will be £120. Cheap for people to call, costs us £60 a quarter with Smart Divert. But where to site it? And what happens when they move?

The alternative is a 'non geographic' number – 0800s, 0845s, 0870s etc. These can be redirected to any 'real' phoneline.
(Mind you, if you want to change where you redirect them to, you can end up paying £25 or so a time, so don't even think about trying to do away with Smart Divert.)

As Arthur C Clarke predicted decades ago, the real cost of calls is now tiny and independent of where they end up. The fact that you pay more for eg long distance calls than local ones is purely down to marketing and companies charging what the market will bear. So it's not surprising that various sorts of lines are available.

0800 numbers are free for callers (unless they're using most mobiles) but we'd end up paying the cost of the call. Exactly how much depends on who sells you the number — up to 12p a minute. Most providers charge a 'rental' fee as well (£60 a quarter with BT, for example). As a helpline, we'd qualify for an 0808 number — still free to call, but it'd cost us about 4p per minute to receive calls. No thanks.

0870 numbers are meant to cost people with BT lines the same as a 'national [non-local] call', but they're much more expensive with other providers. (There are people who think they should be redesignated as 'premium rate' lines.) Because they generate piles of cash for the telecoms company, they're prepared to share it with you. BT, as ever, is the meanest, but finding someone who'll give you an 0870 number for free and hand over 1p per minute every time someone calls it is not difficult.

It was tempting, but not for long. For example, BT customers on the 'BT Together' scheme (which experience tells me they are put on whether they request it or not) are charged 1p (rather than the usual 1.5p) per minute for evening local rate calls and 2p (rather than 4p) per minute for national calls. Except that 0870 numbers are still charged for at the 4p per minute rate. Not good.

There are also lines that cost the caller even more – like 0700 numbers – which are so profitable that it's hard to move without suppliers begging you to have one, but that was never a realistic option.

In between 0800 and 0870 is 0845 which for most people costs the same as a local call, even with BT Together. As there's less money to go around, many suppliers (including BT of course) expect you to pay to receive calls on an 0845 number. With many suppliers, the difference between receiving an 0800 call and a 0845 one is pretty small. In fact, some suppliers will charge you more for receiving off-peak 0845 calls than peak rate 0800 ones! Fortunately for us, there are some suppliers who don't charge to receive 0845 numbers.

So we've gone for an 0845 number from one of them. 0845 450 1263 in fact. This costs us £120 per year in rental, but nothing per minute. I'm not entirely sure if the supplier is going to make any money off us, but at the moment I don't care.

Calls to it end up on a business line (you can't have Smart Divert on a residential line, so rather than pay £120 for a new business line, we've converted someone's home line to a business line. At least that bit was free.) We're going to pay to have an annoucement of the new number when someone calls the old number over the next year.

And we're going to use all this as an opportunity to have a bit of a publicity campaign.

The end result is that the next year will cost us about twice what it otherwise would, and subsequent years will cost about 50% more. But at least we shouldn't have to go through this ever again.

Oh, some of this will take a few days to work through, so don't try calling it just yet. But it should be up and running by next Tuesday, so we'll only have missed one shift – Saturday lunchtime's. And that's not too bad, given what I've been through to make it so.