My ex is in Spain and is currently using an English moby. If I have to text him, I just text as I would do, were he in England. I've never had a problem. Perhaps for your friend to use his/her phone in Egypt, a code is needed.
When I`ve texted a UK mobile in another country, I`ve just texted the normal number. The Egyptian codes wouldn`t work because the phone doesn`t know it`s in Egypt. Maybe there is a temporary problem with your supplier, or maybe the mobile company of the recipient doesn`t have a roaming agreement with the Egyptian mobile companies.
Joko, it will depend whether your friend's mobile operator recognises Egypt as Europe or the Middle East. I'm with 02 and we have activated European coverage and that covered Morocco when we were there in the summer - but some providers only cover UK, or UK and Europe, you have to actually tell the provider that you are going outside your normal range and ask them if they have coverage in that country and if your friend's contract allows coverage. If not then you may not be able to contact them while they are there.
PS - if it works then you don't need to change the number at all, just use the number as you would in the UK, as others say - the network will convert in, you don't need the Egyptian code unless you were ringing a landline - which you're not.
Your UK network providor will have an agreement with an Egyptian network providor and the text will bounce from the UK mobile mast local to you, ten to the satellite then down to the egyptian network mast, that is where the extra cost comes in, as the egyptian network will charge the uk netwirk for the use of their mast, so the recipient of the text will bear that cost.
These are the words I couldn't find last night - International Roaming. If your friend hasn't activated IR with her phone service provider, then neither text nor phone calls will get through - and not all handsets work outside Europe. If she has, then bear in mind that texts can take several hours to get through, too. Have a read of this, re IR: http://ukinegypt.fco....t/using-mobile-phones