Just been checking over my info and tried to put in my landline number (yes I still have one) as an alternative one in case my phone is lost or stolen but it'll only accept numbers beginning 07.
Why is this?
The real reason is probably that giffgaff will be sending texts to the number, not calling it. However, landlines unless specifically disabled can accept text messages (they are delivered verbally) so the more truthful explanation is that a programmer needs educating to know that landlines are also valid telephone numbers.
@techiebloke Landlines are not as versatile as mobiles. Generally, landlines, unless they have voicemail cannot accommodate service text messages. BT for example will not deliver texts or convert them to speech overnight. Speech synthesis may not be 100% accurate or convey the true meaning as instantly as sms do.
When you are away from home will you regularly check your voicemails, is also a factor.