Try disabling 3G and see if it works then.
Most 'entry' systems using mobiles are (2G) GSM at best, which may also explain why some people mention their 'entry' systems being 'basic'?
Yes, I know, you'll probably want your iPhone to work out for you when you are using an 'entry' system needing 3G disabled, but hey, ain't life a beech? (well, it
sounds like that when some people say it!)
(Yes, that is 'beech' as in 'solid wooden thing to bang head against' and not 'beach' as in 'thing to relax on in the sun'!)